Dreadhorde Arcanist + Buffs. Use Sevinne as an attacker, and use Reckless charge and the like.
Echo of Eons or Commit / Memory: If you have an overabundance of flash and/or instants, and don't rely too much on the number of cards in your graveyard, these can be nice cards. Relying more on your hand than your graveyard / self-mill is a good strategy, but a fine balance needs to be made for sevinne's strong flashback ability.
Increasing Vengeance: Casts 4 times with Sevinne, pretty dope.
Sevinne is a human, so Angel of Glory's Rise perhaps.
Finale of Promise and Increasing Savagery act as huge copy effects from the grave, combined with Pyromancer's goggles, Ral, storm conduit, etc, you've got the potential to copy a somewhat menial spell in an engine-esque way. This should be the flavor of the deck. So Sakashima and Spark are worthy too.Carefully engineer the deck to not do gay (time-taking) things with this feature, focusing more on simple effects like Reckless Charge x 10 or something. Instant speed would be a plus. maybe run Howl of the Horde
Dovescape: Since copies are put on the stack, they'll still resolve. So you could use this as a creature engine, while still playing through it better than other players. Also using Boseiju would be a plus.
Swans of Bryn Argoll *list*: Would be good for Blasphemous Act and any targeted damage. Could possibly be good in teams. Is fun when it backfires. Encourages Star of Extinction, Chain Reaction, etc. Also Stuffy Doll, Creepy Doll, Boros Reckoner, etc, with Darksteel Plate all contribute to this combo, as well as mass damage being a-okay for a Sevinne in play. Self mill goes against this strategy, as these permanents aren't easily brought back, but Sevinne's Reclamation is good with any 3cc or less.
Zur's Weirding: Possibly good due to large number of flashback, but would be more ideal in a deck that truly plays underneath it.
Pariah: Great with Sevinne, and great for damage redirection in general, and can combo with the redirection creatures. So probably a mainboard.
Flaming Gambit: Would seem to be bad, but with enough board wipes, this serves as one of the few flashbackable X spells. Also, if it's copied several times, their creatures will likely die as the copies resolve, resulting in them having to take it directly.
Strionic Resonator: Maybe, cuz it can copy sevinne's ability, but it'll only be good if there are 20 or more cards with triggers to copy.
Flaring Pain: Bad because it'll kill sevinne, but good for blasphemous act in case they have protection.. but it doesn't get through totem armor or indestruct, so it's probably not worth it.
Gaze of Justice: Good only if you have 20+ white creatures.
Geistblast: Swanky copy affinity, good with self mill, probably mainboard.
Grasp of Phantoms: Really annoying to get a creature placed back on your library, but this card might be too mana intensive to run.
Goblin Dark-Dwellers sorta good, but depends on the mana cost of the spells in the deck, and how good they are. probably won't use.
Spellskite: Probably use, but depends on the permanents in the deck.
Gravitic Punch: Would be good if you plan on having high power creatures, works in tandem with Reckless Charge.
Ignite the Future: Best with big mana cards, can "play" land cards, and synergizes with exile-getback. Could result in a big play lategame when flashing it back. Goes against X spells tho. so + to the starfall mogg combo. Nope, doesn't go against X spells, so is just simply good. You have the option of playing them OR casting them for free.
Brass's Bounty, Spell Swindle, etc: The bounty especially contributes to the "big mana" requirements of the deck, so this is an include.
Mystical Tutor: Good for increasing the consistency, especially when you need mana, grab a Brass's bounty.
Bosium Strip: Good for flashing back things like Brass's Bounty or any godly card. Gets better the more godly cards you plan on having in your graveyard. You must weigh the native flashback cards vs these godly cards when engineering the deck (designed tier: half-step above premade, tyler's madness)Dockside Extortionist can contribute to big mana too.
Sevinne's Reclamation works on Dockside Extortionist.
Pirate's Pillage: Draws two, gets a discard done, and gets 2 treasures. Pretty dope.
Smothering Tithe: An auto-include for big mana, is helped by academy rector, idyllic tutor, but hurt by self mill. Distinguishes this deck from the Niv spellclone deck.
Extra turns: These will simply up the powerlevel of the deck, especially being copiable from the grave... Could make this a win con, but for powerlevel concerns extra turns should probably not be used.
Increasing Devotion: Big value card. sorta a win condition. Might consider running Anger.
Insult / Injury: Awesome for burn. With a decent flashback. It would double/quad/x8 spells.
Izzet Chemister: Amazing card! Core in a self-mill deck, and can cast big value spells early. Sevinne's reclamation can reanimate him. So can Sun titan.
Jaya Ballard: Too flavorful not to include, as all her abilities scream out the functions/purpose of the deck
Jace, Vryn's Prodigy
Flip: Draw/discard is nice, and he'll flip quickly into a free bosium strip card. Might add.
Vedalken Orrery: Might run this, because using Sevinne each turn would be nice. Also responding between activations of timetwister.
Leave / Chance: probably won't use because the first mode has no practical value in the deck, only niche value, such as when your guy gets stolen.
Lightning Surge: Big damage from the grave, and if you copied it a few times it'd be painful, but not the end of the world for people.
Metallurgic Summons: Great with Smothering Tithe, Brass's Bounty, etc. Getting all shit back to hand can be devastating. But might wanna just focus on the more trivial return spells.
Mizzix's Mastery: Great for big bomb spells, and in general acts as a 4-mana cost flashback-and-play, so it's probably worth it. Plus, you're "casting" the copy, which can be good (or bad). Would be crazy to copy it several times, overloaded, thus casting your whole graveyard several times (without paying the cost) However, it doesn't synergize with Sevinne, because the card isn't being cast from the graveyard. Still worthy of maybe maindecking.
Onward / Victory adds to that beatdown aspect, which is cool. Also is team friendly. Is especially cool because it doubles itself, which is the theme of the deck.
Eternal Dominion: If you can copy this about 3 times, this can be called a win condition. Especially vs stuff like obliterate and game resets. Cool flavor, too. Synergizes with activated abilities.
Past in Flames: Super expensive, but useful from the grave, since our toolbox of flashback-makers for non-native-flashback bombs is pretty slim.
Quiet Speculation: Good if your flashback cards are super useful, toolbox style. With past in flames, they kinda are.
Runic Repetition and Pull from Eternity: These are both good, probably included, depending on how strong the flashback cards are.
Prismatic Strands: good, but must guarantee a white creature
Recall: for 5 mana, you discard 2 and return 2 of ANY CARD back to your hand. Might be worth it depending on how important discard is, and how important getting cards back is.
Refuse / Cooperate: Really nice for copying spells from the graveyard, #copytek, but also could be interesting to copy it's first ability multiple times in response to someone's big x spell. Core.
Recoup: Might need, to guarantee flashback of big sorc bombs. Overall looks weak on the surface though, but could be a diamond in the rough, as most flavor cards are.
Runehorn Hellkite: +1 creatures, +1 self mill, +1 draw
Seasoned Pyromancer: +1 creatures, +1 draw/discard... he's 3cc with 2r, for draw two discard two. Works with sevinne's reclamation, but maybe too weak to include.
Seize the Day: Great for the beatdown aspect of the deck. Also, triggers anything that comes with the combat, such as God-Pharaoh's Gift. Might also run Neheb. Also, is a team friendly card. Core.
Shreds of Sanity: it's red, so pyromancer's goggles copies it easier, and it's only 3, with 2 colorless. nice.
Silent Departure: Bounce tek.
Torrential Gearhulk: A must, due to the absolute synergy.
Throes of Chaos: Hits that nice 3 cc sweetspot, and has the potential to be a diamond in the rough.
Nin, the Pain Artist: Synergizes perfectly with Sevinne, and also with swans / mogg creatures. Gets back with reclamation and Sun titan.
Nivmagus Elemental: Useful for stopping people redirecting big spells, or just "backing out" in general.
Final Fortune: Since I can run arty fetch, I can get the Sundial of the Infinite. Seems decent, but probably a bit "much", since these spells are absolute trash unless you have that artifact.
Goblin Engineer: Synergizes with treasure tokens, and is good for reanimating valuable artifacts. Or putting something like Soul of New Phyrexia into the graveyard.
Fabricate, Gamble, etc: Of course tutors are great, especially with flashback and copy effects. Multi-fabricating was great.
Galvanoth: Only good if you always want to cast whatever you reveal. Might be an include. Same with God-Eternal Kefnet.
Goblin Flectomancer: No, because he can be reanimated and used against you. Willbender is much better. Spellskite can be used against you also, so maybe not him either.
Invert / Invent: Tutors two inst/sorcs, and doesn't get exiled, so can flash it back. Strong, but particularly if your deck has all of it's tools in it's inst/sorcs.
Jaya's Immolating Inferno: We need some mass-damage spells, capable of killing all opponents at once. Also these become broken in teams. Fall of the Titans.
Mission Briefing: Yes, it's a flasherbacker.
Narset's Reversal: Doesn't save a flashback card from exile. It says "if it would leave the stack". However, alternative flashback abilities like Bosium Strip & Finale of Promise & Finale of Promise & Jaya Ballard & Mission Briefing & Torrential Gearhulk are compatible. Exceptions are Jumpstart and Flashback due to the specific rule wording of "If it would leave the stack, exile instead of anywhere else"
Niv-Mizzet, Parun: He's so strong that he's tempting to run, but his colors are difficult.
Precognition Field: Good, like a cheaper futuresight
Radiate:
Doublecast, Repeated Reverberation, Howl of the Horde: These can all be strioncally resonated.
Chain Reaction: No, because it's value is determined upon resolution, so copying it isn't that good.
Scribe of the Mindful: Rather than Archaeomancer, this is a better option, as Sevinne's Reclamation can reanimate it, but it's a little slower.
Soulfire Grand Master: Good spell recursion, but moreover the lifelink. Also it's a Human, and also the colors match sevinne's identity (swag).
Jeskai Ascendancy: Matches sevinne's identity, and does cool stuff. Engine pieces do feel shitty to run though, so might not include.
Taigam, Ojutai Master: Uncounterable is cool, and so is rebound. But rebound doesn't work with Flashback.
Trail of Evidence: Good, can add to an enchant/aura fetch sub-meta (by recurring idyllic tutor and fabricate.
Conflagrate: Expensive to hardcast, but cheap to cast from the grave, and generally a good discard engine from the graveyard... this is it's utility. Also if you draw tons of cards, like with Wheel, or just mass draw, you can do a lot of damage for basically 2 mana.