I am writing this article directly after playing the Ikoria Early Access event, and after playing with companions, it is clear that they will completely change the way a game of Magic is played. This isn't necessarily a bad thing, it is just different than what we are used to. I have already heard statements about how companions are ruining the game, and while I don't think this is the case, it also seems far too early to draw a final conclusion. In my opinion, companions can be a good thing, if their power levels are accurately managed.
It is hard to say exactly how many decks in Standard will play a companion, but I could see it becoming well over 50% very quickly. It is still difficult to say, because the idea of having a companion is all so new. If you don't like the idea of having a card in your sideboard that is essentially an extension of your maindeck, imposes deckbuilding restrictions, and is also more or less a free spell you don't actually have to draw to cast, you will not like companions. For those that don't like big changes, companions will not be for you.
Let's go ahead and talk about some of the high-impact ones, and where they might fit in Standard.
Umori, the Collector provides a reason to play only creatures in your deck, and for a deck like Mono-Green Aggro, or some Gruul decks, that sacrifice will be worth having access to Umori as a four-drop each game. Umori doesn't look like one of the more high-impact companions, and it isn't, but that may not stop it from seeing a decent amount of play. The key will be if decks actually need to make big sacrifices in order to play it.
The deck is very cookie-cutter, but it gives you an idea of what Mono-Green might look like.
Keep in mind that when playing a companion, because of the imposed restrictions, your deck is more likely to just have four of all the best cards (unless there is a deckbuilding restriction on play four-ofs.) The companion essentially cuts down the card pool. In this case we only have green creatures to choose from, so the question becomes whether there are enough of them out there to build a strong deck.
Yorion, Sky Nomad is definitely one of the most hyped companions, and for good reason. Adding 20 extra cards to your deck is well worth getting access to this card. We will be seeing a lot of 80 card decks moving forward.
For reference, we have seen competitive Battle of Wits decks win tournaments, and those strategies played north of 200 cards. Adding 20 cards, especially to a three-color deck, is very manageable. It is nice if you are able to take advantage of the flicker effect to make Yorion better, though it still could be worth it for decks to want Yorion as a 4/5 flash creature, and that's it.
So far Temur Elementals has been the most common archetype to abuse Yorion, because of how well Yorion works with enters-the-battlefield effects. Crokeyz was seen playing this, and he has become one of the top deck builders for Standard play:
This is a good first shot at building an 80 card deck. I suspect the spell-to-land ratio is a bit off (I would likely play a couple less lands), but regardless, get used to looking at decks like this! Having Ketria Triome provides a cycle land which is certainly valuable. The deck could probably be built in a way that is a bit less heavy on Elementals in exchange for more generically powerful cards like Hydroid Krasis. The issue with trying to be a synergy deck like Elementals is that it becomes increasingly difficult to maintain your synergy as more cards are added to your deck.
For example, by adding the extra 20 cards, fewer of your draws involve multiple Risen Reefs. We may start to see a migration towards more generic "good stuff" decks that have 80 cards in them, and I really don't think that is unreasonable. The good news is that there are a ton of ramp options, so we see a lot of redundancy here in terms of ways to accelerate. In other news, mill decks have an uphill battle, as some decks will have more cards in them than before!
Lurrus of the Dream-Den can very well be the centerpiece to an aggro deck. Not having a top end in terms of permanents definitely is a downside, and will make a deck built around Lurrus quite linear. Decks will be all-in aggro, and very vulnerable to sweepers. However, getting a free and powerful three-drop is very important. Realistically you can play three creatures on the first two turns, and follow up with Lurrus.
It is a matter of looking into what high-quality creatures to play alongside Lurrus at the one and two-drop slot. Staying Orzhov is one option, but theoretically, you only need white or black, and not necessarily both. Keep in mind it is possible to play three-mana cards that are noncreature spells, so you can still have some top end, as long as they are not permanents.
One direction to go in is to actually build a Human-based deck. I stumbled upon this Mardu list that is an interesting idea:
Getting to play with Lurrus has a pretty big cost here, and this might not be the right direction to take the deck because you can't play with General Kudro of Drannith. However, getting to play Dire Tactics might be enough of a reason to want to be Human based.
It is certainly possible to play a version of this deck without Lurrus and with General Kudro. We will see a variety of different Human-based decks in the early going of this format.
Here we have another hybrid companion, which means there is a ton of flexibility with what colors you play with Lutri, the Spellchaser. This is another case where you are likely to be a generic "good stuff"-style deck rather than synergy based. The reason of course is that you can't play more than any one card.
Lutri is a companion that will get better the larger the card pool is in Standard. There will definitely be players who try to figure this one out.
I think Obosh, the Preypiercer is much more likely to see play in Standard compared to Gyruda, Doom of Depths. Having odd casting costs like one, three and five is more realistic compared to two, four and six. Obosh likely ends up in an aggressive deck with a lot of one-drops, a couple three-drops, and then Obosh as your five-mana payoff if you can get there.
Kaheera, the Orphanguard is the last companion I'm going to be talking about, and should be one that sees quite a bit of play. There are really a few different directions you can go with Kaheera, as Cats, Elementals, Nightmares, Dinosaurs and Beasts actually encompass more creatures than you might think. You can potentially go toward a tribal strategy, but it isn't necessary. There is the possibility of even being a control deck with only a few creatures in it, as there are a variety of strategies that can realistically build around this restriction. Keep in mind there isn't an amount of creatures you need to play, so (for example) last year we could have seen Kaheera somewhat randomly in a Nexus of Fate deck.
Here is an Abzan deck that should provide an idea of some of the various creatures you can put into a deck with Kaheera:
There is a lot going on with this deck. Yes, we have the tribal synergy in order to play Kaheera, the Orphanguard, but there is also a toolbox based around Fiend Artisan, and an attempt to put cards in your graveyard to make Fiend Artisan better. I know that there are plenty of players out there looking for Abzan decks to make their triumphant return to glory with Ikoria, and Fiend Artisan has a lot of potential. There are some really efficient removal spells in Ikoria, and of course Mythos of Nethroi is perfect for Abzan colors.
The lists I have presented here are very early on in this rapidly developing Standard format, and are meant more as ideas to build upon than final products. As we move forward in the next few weeks, these ideas will certainly become more fleshed out. The deck-building restrictions on most of the companions feel like significant sacrifices to make.