While so many things have improved in Rock Band 3, there's a few simple things that could make the drums so much better. Maybe they can work on these for Rock Band 4; goodness knows they're going to be hard pressed to think of anything to add after the tour-de-force that RB3 turned out to be.
Tempo Changes: The only indication when a song's tempo is about to change is the spacing of the notes and the pale white lines changing. That's enough for playing guitar or keyboard, but not for drumming. If you're familiar with the song you might not need to be warned, but if you're not, it's really important. It'd be a great improvement if any tempo change came with some kind of visual warning, like a blinking red bar, for the drummer.
Fill Sounds: On RB3-native songs I've noticed that the sounds the drums make during fills aren't always the same sounds; some songs have a more electric-drum-set sound, for instance. That's a step in the right direction compared to previous versions of Rock Band, but it's not enough. The fill drums still often don't sound like they fit the song. How hard would it be to use samples from the actual song?
Cymbal Riffs: Drummers probably have a term for this but I don't know what it is. You know how sometimes when a song ends, there's a sort of soft sussuration on one or more of the cymbals that just trails off, somewhat arhythmically? When these show up in Rock Band songs I hate them, because they get reflected as a string of notes that are not in any rhythm that bears any resemblance to the song itself, and it's nearly impossible to hit them. You have to play the screen, not the song, and it's so arbitrary and hard to hit. It bears no resemblance to what a drummer would be doing at that time. Get rid of them: replace them with something like a fill or Big Rock Ending just for the drummer, when only cymbals can be played. Or just omit them if that's too hard.
Omitted Notes: When you're playing on Easy or Medium difficulty, a lot of the actual notes in the song get omitted. Sometimes, though, it'd be easier to be able to play them because they're so compelling (for instance, on those long tom-tom runs in "Sister Christian" where towards the end there's that extra note in the middle, twice). Other times, if you're feeling adventurous, you think you can fit the extra notes in. It would be nice to have an option where, if you played extra notes that precisely matched what would be present in a harder mode, even though they weren't displayed, you at least didn't get penalized, or maybe even got some extra points.
Fill Scores: I'm not sure how possible this is from a coding standpoint, but I would sure love it if the system somehow identified a good fill from a bad one. I really try to make my fills fit the song (and if it's a song I know, I often try to play the fill that the original recording includes -- when you nail that, that feels good). But if you just want points, the best thing would be to always play red-yellow-pause-crash and pay no attention to the song or its rhythm. Wouldn't it be nice if the game at least made some token effort to identify if the notes you're playing are at least in time with the song's tempo? Sure, making the computer measure the artistic quality of your fill is impossible, but it could at least reward you for keeping the song's tempo, and maybe a little more for playing several pads instead of just one or two.