Since I cannot post the "Tai has two hands" meme here: how would a polyship with Matt and Izzy work in your opinion?
Ooof, yeah, good question indeed. Since I've just talked about how difficult it is to make their platonic group dynamic work - despite them being one of the most obvious/prominent trios -, making it romantic may be an even greater challenge. However, that's the fun in talking headcanons after all!
I believe the main thing we really can agree on is that Taichi would be the center of it all. The problem with these three (and Adventure's characters in general) is that they aren't very good at dealing with their feelings. Facing and confessing them may be an even greater beast. So what I could see with this trio is lots and lots and lots of subconscious yearning (on Yamato's behalf) and unrequited pining (on Koushirou's behalf). Yamato may take ages to really admit that what he feels may be romantic attraction, which is, in my opinion, why he tends to react a bit more aggressively towards Taichi without wanting it - he's mostly frustrated with himself. With Koushirou, I like to headcanon that he realized his feelings quite a while ago and never acted on it - not damaging your most important friendship when you think you have no shots anyway is way more important.
Taichi may be more on the oblivious end - and I think he would only really start thinking about it if he got directly confronted by it. He has strong feelings and attachment to both of them - but probably never thought about them from a potentially romantic (or even sexual) angle for a long time. (Demisexual!Taichi ftw!!! The thing I headcanon with him is that, once he HAS his feelings figured out and confessed, he is incredibly physical, affectionate and initiative - but NOT before that.)
So what if a confession does happen? How would it happen, who would initiate it first??? ... Those are all questions I haven't ever thought of. I do believe that Yamashirou's dynamic plays a big factor in this in general; their novel versions for example might actually compete with each other over Taichi's attention, whereas their anime selves (and mostly every other versions of them) would come to a quiet agreement not to intervene with the other and let Taichi make the final decision. However - it would be incredibly funny if, somewhen in their mid/late twenties or early thirties, they made a pact to actually go for him at the same time. Actually propose something along the lines of a poly-relationship (even if the two of them are not exactly romantically or sexually interested in each other yet, the mutual respect and appreciation for each other may be enough to agree on wooing the person most important to them together). I could actually see them as room-/flatmates - so something like that may come up eventually! And Taichi would be caught off guard COMPLETELY... But they would (awkwardly) try to give it a shot and make it work step by step! (On a very different note, I may still have to get used to Beyond!Taichi feeling a lot more confident and forward again, so maybe there actually IS a version of Taichi who boldly makes the first move there!)
Another variation of this polyship may involve the "two of them are already together and invite the third one in", which, for me personally, would most likely be "Taishirou already dating and letting Yamato move in after returning to Japan after a longer a space mission, so they'd have to figure each other out in completely new ways". It's similar to how I would imagine Taikoura to go down - where Sora was kind of a stray cat taken in by Taishirou, discovering their feelings along the way. And I think the same works incredibly well with Yamataishirou!
A third scenario that is more of an AU idea I haven't fully fleshed out yet would be making these three work partners - or as Takeru would probably approve of, making them work as undercover spies. (I still believe that all three of them may have started working under Daigo and Maki if Tri didn't end the way it did...) Having Taichi as the charismatic lead strategist, Yamato as assassin gun specialist and Koushirou as expert hacker, being chosen to work together on missions, even though they couldn't be more different from one another, but slowly but steadily falling for one another. (Like, if you wanted, you could just assign Jock to Taichi, Nerd to Koushirou and Goth to Yamato and have all the tropes you need. XD) Or... Just gimme my Totally Spies AU for them!
