I want 343 to make a Mass Effect style RPG in the Halo universe.
Or maybe since 343's forte is FPS, contract the series out to a top-tier WRPG developer (or make a new 1st-party dev to make such a WRPG) and have 343 act as a consultant, while 343 continues to make the mainline FPS games in the Halo franchise.
A Mass Effect style RPG in Halo's universe would be so incredible. I'm imagining being in a big ONI/UNSC facility reading hundreds of condex entries on the myriad of fascinating characters/places in the Halo universe (like the Citadel/Presidium in Mass Effect), i.e. all the spartan programs, all the Halo installations and each of their monitors (so 343 Guilty Spark, + Penitent Tangent, etc...), Halsey, Planet Reach, the Covenant Council, ODST training, etc...
You could visit Halo installations not to play a traditional Halo-style FPS action level, but you'd visit a Halo and spend 60 minutes there just wandering around in its Library, talking to its monitor, doing a side quest for some ultra-religious Elites/Unggoy on the installation, etc....