Interesting build. My immediate thoughts on sideboarding are:

Pact of Negation to protect the combo, deal with other combo decks.

Terastodon? to deal with Liliana, Karn, etc. (though Beast Within might be better against things other than Tron).

Firespout to deal with weenie decks. It will often have the side benefit of allowing you to keep some of your mana dorks.

Nourishing Shoal to deal with problematically fast decks - esp. Burn.

@formayor I completely agree with your suggestions of Pact of Negation and Nourishing Shoal.

Terastodon doesn't really have enough impact to be considered, he is good but also weaker than a Progenitus or Worldspine Wurm. Beast Within may be useful, but I feel that it doesn't have a great enough impact to justify watering down the combo.

Firespout could be useful, although its a bit of a toss-up between that and Pyroclasm.

Thank you for the suggestions!

No problem, glad those work for you - The reason I suggest something like Terastodon or Beast Within is that when I use d to run a Polymorph deck with Worldspine Wurm, he would often end up sacked to Karn Liberated or Liliana of the Veil when I didn't have anything else on the board. Same would be true for Progenitus in your case. My solution was to sideboard Terastodon, who pretty much instantly beats Tron upon resolving, and causes a gigantic setback for BGx if you hit something like Lili plus 2 land, or just 3 land works too. Granted, he's not good overall - and shouldn't be mainboarded - he's highly situational. You should be less worried about Lili with mana dorks ready to sac, but chump blockers also buy you a turn to deal with the tokens Terastodon makes. I guess what I'm saying is, don't think of Terastodon as an alternative fatty, think of him as a one-sided Armageddon with a minor elephant-token downside.

@formayor I do see your point. But do you think that Terastodon is superior to something like a turn 3 Crumble to Dust? It's much easier to cast, and likely can keep them off Tron for long enough to win.

Nice deck! Do you think it could use Tooth and Nail?

@TheLivingCME Tooth and Nail requires a much more dedicated ramp package, and it normally can't get creatures into play until turn 4-5. I'm trying to stick a creature earlier by getting it out on turn 3. Basically I'm trying to brew a new deck rather than just rehash the Tooth and Nail decks.

Crumble to Dust is clearly the better play, and it's basically a silver bullet against RG Tron (not so much U Tron). That said, unlike Terastodon, it can't be tutored by you deck, which means it probably needs 2 SB slots at least. So the real question is whether or not CtB is any good against other decks out there - to justify the SB space. It seems to me like it will be decent against 3-color decks generally, but not a flat-out killer. So it's a tough call.

@formayor You convinced me. Just added a single copy.

Looks like a very good starting point to test/tweak your sideboard. One note: though it has a small downside, you might find Creeping Corrosion useful, as your deck might more consistently be able to drop it on turn 3 (and that really matters vs. Affinity). Just something to keep in mind.

@formayor I had forgotten it exists. Thank you for all your help on the sideboard!

Also few lists run Welding Jar, so it's not likely to be an issue.

I worry about your fatties. They're decent, but I would opt for more utility out of them. They're very hard to removal, which is great, but they don't help against, say, burn, affinity, infect, etc. And with a deck that doesn't have any interaction, I'd say that's a bad thing. Think Thragtusk, Hornet Queen, Wurmcoil Engine. These are all really easy to hardcast as well, which may be good or bad. But they have better utility in my opinion so you don't lose to burn spells the turn after landing Progenitus.

Good luck! Awesome deck! It did a turn 4 2x worldspine wurm in my playtests.

@Figag The reason why I opted to use Progenitus and Worldspine Wurm is that they are better than anything else out there in terms of sheer power. As long as they stay on the board, we are guaranteed to win.

I have been considering swapping the Kiora, Master of the Depths for some Huntmaster of the Fells  ^Flip or Thragtusk for just value creatures. The deck has only about 4 flex spots (two dorks and kiora) I'll do some playtests and see how it goes.

Thanks for the feedback!

Edit: I forgot to mention, but our match-ups against linear combo decks are really rough pre-board. That's why we have 6 counterspells and other interaction in there, just to try and have some game against them.

I'm not sure I agree that as long as they stay on the board that you are guaranteed to win. Note that they each take 2 turns to kill the opponent. Against, say, BGx, yes they auto-win when they hit the battlefield. But remember the aggro decks. Almost any aggro deck can just shrug their shoulders at the sight of a Progenitus. They simply just keep swinging because they can go around your dudes. Note that turn 3-4 is when your deck drops a fatty. You drop Progenitus turn 4, attack for 10 turn 5, then turn 6 you have the kill. By turn 6, Progenitus or not, Zoo, Infect, Burn, Affinity, Merfolk, and Infect will all kill you because they don't care at all if you have a fatty. It's life totals and interaction that win the game vs them. Having creatures with amazing utility is important. Of the above list, all but Infect will likely lose if you drop a Wurmcoil Engine or Thragtusk instead of Progenitus.

I don't have enough of a feel for Kiora yet.

I'd consider running something like Boseiju, Who Shelters All because it will dramatically improve your control matchup. As for combo decks, counterspells, sadly, really are your only option.

Good luck!

@Figag I do agree with what you are saying, and pure linear aggro decks can be rough. I've got in some playtests against aggro decks, and typically if I can turn 3-4 a fattie then I am still ok. Burn is extremely rough, but we specifically sideboard Nourishing Shoal to combat that.

Playtests with Thragtusk have been pretty good. I'm not 100% certain if he is better than Kiora, but he is certainly powerful and a good option. I'll probably post an update when I have made up my mind.

Wurmcoil Engine sadly can't be used here, since it can't be Dramatic Entranced. It could still be hardcast, but I'd rather have all my creatures be cheatable.

As for Boseiju, Who Shelters All, I cut them out of the original deck idea because the pain from using it made our deck lose more frequently to the same aggro decks, and we don't have enough sideboard space.

Thanks again for all the suggestions!

I have a buddy who plays a similar type of deck and he uses Unexpected Results and Kozilek, Butcher of Truth/Ulamog, the Infinite Gyre

@Gaunth Unexpected Results seems interesting, but it isn't ideal due to the fact that I wouldn't have high odds of hitting a fatty. Still a fun card, but not exactly competitive for Modern.

The Eldrazi titans have the fatal flaw of not being green, so I can't Dramatic Entrance them in. I tried out Painter's Servant to get around that, but it just wasn't worthwhile. Even in my janky decks, some things are just too janky to work.

Hey! I love the deck, and tried out a modified list on a playtest website I know of (Untap.in.v2) The direction I went with it is "How fast can I get the job done without less moving pieces?" So I went a heavy land, play Summer Bloom route, and I don't personally play through the breach, as I opted to play blue instead of red so I could use Serum Visions, Vendilion Clique and Spell Snare. My creature base is Progenitus, Worldspine Wurm, Birds, Clique, and Primeval Titan. Don't know if you'd be interested in this route of play, but I'd figure I would comment that you inspired a different list, even if it doesn't closely resemble your own. Good deck! upvote from me.lemmingllama

@C.LewisMTG I used to run Temur colors, because Serum Visions was so powerful as a way to set up cards. Also the sideboard options are much more robust. However, it also slowed down the deck, and was a little too expensive for me to try and put it together in real life. If you look at the revision history, you can find the original version of the deck.

Summer Bloom seems interesting, but what do you cut for it? If you are just removing mana dorks, then I can see your deck being extremely slow if you don't draw a copy. Do you have a list on tappedout I can view?

Either way, I'm glad to hear that you enjoyed my list and it inspired you to brew some. Modern is a wide open format, so brewing is honestly too much fun. Also thanks for the upvote comment, even if it didn't translate to an actual upvote of my deck.

Oh god I'm sorry, I was looking back and fourth from this page and my list on the website, and it completely slipped my mind to actually hit the upvote button lol. and I haven't posted it to Tappedout yet, but when I refine it a bit, I will definitely let you know (: lemmingllama

Great. Just tag me whenever you put it up and I'll be sure to give it a once over. Although I haven't had the greatest success with this deck against top tier decks, my playgroup is also about 100% Delver and Twin. Trying to make it work out nicely.

List is up (: lemmingllama

Might I suggest replacing four of your ramp creatures with Devoted Druid? Having 5 mana on turn 3 seems a lot more important in this deck than having 3 mana on turn 2.

@dragonestea That is an excellent suggestion, I'll certainly be adding at least two immediately. Not sure if it is worthwhile to cut the Arbor Elfs and Birds of Paradise since they also color fix as well as ramp, but the Llanowar Elves can go

I don't think you really need the extra fixing from Arbor Elf. Excluding Progenitus you don't have any spells with an RR in the cost.

@dragonestea Arbor Elf can sometimes ramp harder than Birds of Paradise. I've done some testing, Devoted Druid is pretty great. I swapped out my fourth copy of BoP for a third druid. Likely not cutting any other dorks for the fourth copy though.