Just get rechargeables.
If RB4 happens, the ability to extend the game through DLC Tour Packs is definately up there on my list of wants.
They could add a better tour mode, maybe even an RPG-style thing where you choose where your band goes, whether you become superstars or underground hits. Maybe they could introduce new game modes and party options, much like how Guitar Hero 5 tried, but failed with the Rockfest multiplayer setups and instant gameplay on the start screen. Maybe Rock Band could feature new ways to play.
And even with Rock Band's extensive soundtrack, we still have some really great, on-disc soundtrack worthy songs we can get. I'd some, but I don't want to take up even more time.
The way they're doing keyboard is a great start. However, restricting it to one hand kinda takes a but of edge out of the experience. Maybe they could come up with a system for two handed charts, though this is unlikely due to backward compatibility issues.
This isn't a great list of things Rock Band 4 could do, but it's a start.