I think Halo 4 and 5 have been the best Halo games of the bunch (4 having the best story, and 5 by far the best multiplayer (ignoring lootboxes in warzone)), but strangely, the fact that I like 343's work so much is the reason that I think I will be disappointed.
I don't think 343 will just "take the good parts from 4 and 5 and make a perfect Infinite", because half of the community hates both 4 and 5, for the reasons that I love them.
I think split-screen is good for everyone, but I feel like there's a good chance we get thrust back into the vapid narratives that were pervasive throughout Halo CE through 3, and, although I'm sure the multiplayer will be fantastic in terms of balance and the likely true return of BTB, I doubt it will have the movement mechanics that I think flourished in Halo 5, and that's going to hurt. Especially if they decide to steer back into (what I perceive to be) the skid of Halo 3 (which imo just felt floaty, and had much too high bullet travel time).
I also fear that they will bring the flood back in a major way, and I always thought that having anywhere near as much flood as Halo 2 or CE kind of ruined the fun of a lot of the missions (woo, shooting a bunch of popcorn infection forms, how exciting!) because I see the flood as a pretty boring and cliche zombie enemy (which isn't very dynamic to fight).
I think we will get more open missions (like Halo CE), which will be good in my opinion, but I also think we risk the boring kind of "do the mission in one direction then do it again in reverse" kind of thing that Halo CE was plagued by.
So I guess, weirdly, the reason that I trust them less (to deliver a game that I will enjoy), is because I trust them more in their ability to listen to feedback. Go figure.