DemonDragonJ Cryptolith Rite provides color fixing and extra mana; Joiner Adept only provides color fixing.

In a vacuum, Dryad > Lantern > Cryptolith Rite > Joiner Adept. In the vast majority of situations.

As someone who has been forced against my will by my girlfriend to watch W40K stuff, I'd say the Astartes can be most readily mapped to humans. They're basically bioengineered humans made for battle, that's it. So if you were to map them onto any creature type, Human Soldier or Human Warrior would fit the best, I feel.

Umori, the Collector might enable some combos?

Solid Snake, Master

jk

Dawnrise Nogglecrusher

Creature - Troll Shaman

When this creature enters, if it isn't night, it becomes night.

Sacrifice a creature: Put a +1/+1 counter on this creature. Activate this ability only twice each turn.

As long as it's day, Dawnrise Nogglecrusher is an artifact with "{T}: Add {C}{C}" and loses all other types and abilities.

5/5

Now make Solid Snake.

Yarok, Waste Besieger

Legendary Creature - Elemental Horror

Deathtouch, lifelink

Landfall - Whenever a land enters under your control, put a +1/+1 counter on Yarok.

Whenever Yarok attacks, you may sacrifice three lands. If you do, put X +1/+1 counters on each other creature you control, where X is the number of +1/+1 counters on Yarok.

When Yarok dies, return all land cards from your graveyard to the battlefield tapped.

2/4

We have RGWU, GWUB, and UBRG. Pick WUBR or BRGW to make a Zendikar legendary for next.

Vindicator

Creature - Mercenary

Haste

When Vindicator dies, each opponent creates a Treasure token.

3/3

Make the Elder Guardian!

capwner ah, yeah, missed that part. I'd probably switch to prefer Soul-Guide Lantern then, tbh.

I think Relic of Progenitus is the most well-rounded and versatile of the ones here, as it provides three benefits: card advantage, targeted graveyard hate (against reanimator decks), and mass graveyard hate (against abhorrent oculus-type decks). But any of them are pretty good.

I used to be #1 in both on my old account (and was helper for a short time on this one but never went for decklord again) and never figured out exactly how they were measured, but the basics of it from what I can gather are:

Helper is calculated through how many card suggestions you give to people on their decks that they accept, I think?

Decklord is some amalgam of your general activity including upvotes received, comments posted, and decks made, I believe. It changes a lot more slowly than Helper.

Could be wildly off base though, that's just how I always thought it was calculated.

Hey, I already made both, but I'll redo my Ender Dragon design since it's a little clunky.

The Ender Dragon

Legendary Creature - Dragon

Flying, menace

Dragon's Breath - Whenever this creature attacks, it deals 2 damage to each other creature.

Sacrifice an artifact: Regenerate this creature. Activate this ability only once each turn.

7/7

Make the Wither or the Warden.

I'd definitely say I prefer a secrecy aspect as that's really what makes the card function.

DemonDragonJ I get it unfortunately, blue mechanics are honestly some of the hardest to work with for me as a custom card designer.

Something that I haven't seen mentioned yet that I would like to touch upon is that in general, cards (at least good ones) get more mana efficient as they get more expensive. Take your example of draw spells. Opportunity may not be very useful, but there is a better cost-to-draw ratio for cards like Overflowing Insight, and Finale of Revelation-esque cards have an arithmetic floor for their effects, which also changes things.

But playing anything later in the game comes with higher risk because the boardstate late game is usually going to be much more hostile/high interaction/complex than early game, so higher cost effects even if they have a higher impact are a lot riskier to invest in.

But to answer your question, I'm personally a mana cost focused builder, not necessarily a mana efficiency focused builder. I'll obviously choose the most mana efficient option for each effect at each mana cost, but mana cost in general will take precedence unless there's a truly unique or insane effect I'm missing out on by not going higher (for example, finding a finisher at 10 mana is a lot easier than finding one at 9 mana).

I think Enduring Duplicity is probably a bit too oppressive, but I'm not really sure how to fix it per se without completely overhauling the design. Maybe some kind of buff it gives to counterspells, kind of like Baral, Chief of Compliance? Just a thought

I think maybe the first ability is just kind of weird in that it insta-wins completely based on luck, which isn't normally something that cards do in MTG, at least not as explicitly. I think some kind of effect based on enchantments could be neat, as a possible synergy to build around and since all leylines (except Leyline Axe and Gemstone Caverns iirc) are enchantments.