Gleiphnir

It wouldn't be the end of the world if we didn't get an Avatar. But having a character that embodies the player is nice like they did in FE7 with the Tactician. If they could incorporate that but also not make the story revolve around the pc, then that would be great. S supports are done right when they don't impact the storyline until the credits. Before that it should just be stat buffs either exclusive to parring up, or subdued effects to characters in proximity. Parring up seems to be a new thing that they've embraced and so far I like it, so I think S supports will be easier to achieve by end game.

If they're going to bring back the magic triangle then diversifying the growth stats on mages/priests/druids is a must and characters themselves have to have inherently different stat growths like they do for the physical dmg characters. Otherwise what I think they could do to mitigate some the defense flaws with stacking Magic res on these classes, would be to borrow a piece of the magic system from Dark Souls where certain types of tomes deal physical Dmg instead. And in all honesty, type advantage stops being relevant towards late game anyways. Your characters can only do so much damage, they are still squishy against a Killing Edge Swordmaster. It turns into the same late game the Weapon triangle has, anyone remember Amelia, aka Sonic the General. Even if an enemy could land a hit, they literally did 0 dmg like 2 times with their brave weapon. Shit I happened to use Ike like 24/7 on PoR and leveled him up to 20 way before his forced promotion and he was a god for the rest of the play through. There will still be broken characters no matter what, but that's what makes the game unique.

How about they do it like they just did in the Warriors spin-off? You are able to progress through two branches simultaneously and choose to progress one side at a time or both in tandem. The catch is that you are required to complete both branches in order to progress further. The titles of the branches could be something like Chpt ##x and Chpt ##y and they follow the open map concept that FE8 had at the end of the game, wherein your main protagonists' tokens move independently from each other after advancing their respective chapter.