I was literally making jokes a couple of weeks ago about people thinking Elementals was a playable Modern deck. And now I'm pretty sure I agree with them: Elementals is currently a good deck in Modern.
This isn't the first time people have tried to make an Elemental pile work in Modern. I ran out of polite ways to say, “If you're going to lose to Plague Engineer, you idiots should play Humans!” when I was writing Modern primers a year ago. What changed from that clunkfest?
The same thing changed as with every other good Modern deck right now. You get to play multiple one-or-less-cost spells from Modern Horizons 2. In this case those are free spells, not even one-drops, which is just another level of "What the heck is going on with the cards in this format?" Now that the metagame has evolved a bit, Solitude also plays a crucial role of being a removal spell you can burn on a Turn 1 red creature or save to kill a later Murktide Regent, something I'm not sure any other card in the format does well.
There are two caps on playing a billion Elemental Incarnations that apply to normal people: the fact they cost two cards to cast for free, and the fact they require cards of a specific color to cast for free.
The traditional way around the two-card issue has been using Ephemerate to keep the body around and get another card off the retriggers. That works here, but you also get to abuse Risen Reef to Coiling Oracle off each of your Elemental Incarnation casts. Or maybe you get to abuse free spells to double-trigger Risen Reef the turn you play it, which typically puts you into “Modern is Commander now” mode the next turn.
You also can just get two-for-one value because Fury is an egregious Magic card. If you haven't had the joy or misery of participating in games with this card where the non-Fury player tries to cast creatures, take some time to do so.
As for that other half of color requirements, Elementals aren't super into that whole "restricted color identity" issue.
Even beyond the true garbage-pile mana costs, decisions are made up the curve to maximize the number of cards you can pitch to whatever free spells you cast. If there's a random terrible-looking multicolor card in an Elementals list, that's because 60% of its job is being cardboard with good pips. You even get to freeroll Kaheera, the Orphanguard as your companion for free cardboard-with-colors equity. And yes, three mana to draw a spell with colors is still significantly better than any non-Elemental creature you could play that would break up the companion requirement.
Non-Elementals decks should probably learn this lesson as well, at least in color pairs with playable multicolor cards. Looking at you for that one, Orzhov.
I'll briefly mention the Glimpse of Tomorrow deck that is more Elementals-focused here, but really only to say that I think the nonsense Eldrazi pile Glimpse deck is a significantly better version of that deck. Worrying about chain probabilities and resources is worse than making an Emrakul, the Aeons Torn and calling it a game.
All or Just Most of 'Em?
There's a bit of a split in the Elementals decklists these days between playing the full Five-Color Pile or taking a “conservative” approach and sticking to the Omnath, Locus of Creation colors.
- 4 Flamekin Harbinger
- 1 Reveillark
- 4 Voice of Resurgence
- 4 Unsettled Mariner
- 4 Risen Reef
- 1 Omnath, Locus of the Roil
- 4 Omnath, Locus of Creation
- 2 Subtlety
- 4 Solitude
- 1 Endurance
- 1 Foundation Breaker
- 3 Fury
- 2 Horde of Notions
- 4 Smokebraider
- 4 Lightning Skelemental
- 4 Risen Reef
- 4 Thunderkin Awakener
- 3 Omnath, Locus of Creation
- 4 Grief
- 4 Solitude
- 4 Fury
The relevant debate on the last color is whether you play Grief or not. None of the other black cards the Horde of Notions build plays do anything significant besides being pitch fodder for Grief starts. They're acceptable to cast if you need cardboard to throw around and that's about it, with Lightning Skelemental often resulting in some weird mana constrictions with Ephemerate and drawing Breeding Pool.
Plus, you aren't cutting relevant cards for black cards. Is Runeclaw Bear with a good color identity really worth it?
The rainbow manabase is also just better than the more traditional one, even if you don't have black cards. There's too much merging of weird color requirements that can bite you when you naturally draw your fetchable lands, Primal Beyond doesn't cost you life, and it's not like any of your configurations beat Blood Moon anyway.
Before you ask, yes, this is enough black spells for Grief. One of the lessons of my Grief and Solitude explorations this last month has been that the required pitch count for the Incarnations is way lower than the Legacy Force of Will “minimum” of eighteen blue cards. Part of it is the London Mulligan and the Elemental Incarnation decks being ones that are less worried about card count. Part of it is that just casting Grief is a fine play. And part of it is that in Legacy, the blue cards you're pitching to Force of Will are good things you want to cast, while in Modern, often the things you're pitching to Incarnations suck and rot in your hand until pitched anyway.