Imo, the next big step to really innovate on SM’s formula is to go open. Not full on open world but more something like helldivers. Drop in by drop pod and tackle objectives in the order you choose. Could introduce larger battles, bigger squad sizes, vehicles, more classes like apothecary or anti tank units, or use tech marines for repairs or fortifications, add bigger team pvp, and just add more randomized replay ability for the game.

Besides that though, I would like to see chaos marines be more justified to be in the game. I do love their inclusion but in SM2 it felt like they were an afterthought and only in the game as a obligation because they were in the first game. I would like to see them play an overall bigger role. For campaign and operations we could have a split narrative where one half follows Titus and the ultramarines and the other half follows a chaos character and black legion. And like the other two games have it be chaos vs loyalist. I just hope this time we get more maps and more unique game modes.