Do some people really want Fable to be more mature (depends on what you mean by mature though, as some of the gameplay was pretty mature), realistic, serious, etc? Sorry, but that's not Fable, and I seriously hope Microsoft doesn't do this to Fable. To me, that's like people wanting BGS to copy TW3. No thanks, The Witcher 3 is one of the best games, but they're completely different types of games. They can learn from each other, but they shouldn't copy.
Preferably, and I would like this, is for them to do as JINX said and have PG switch between 2 IPs. They can prove themselves with Fable, and then the second IP can be the mature, realistic, and serious Open World RPG that some on here want. That's completely fine with me. I just don't want them capitalizing on Fable's name and it's nothing like Fable. One difference I would like though is character creation in this one, especially if it's going to be co-op.
I love Fable for its humor (most of it at least), how it doesn't take itself seriously, its lore, its aesthetics, the choices, and its sim-like gameplay. The gameplay can certainly be overhauled, but it still needs keep a lot of simulation elements. That's of the biggest things that makes the series unique, and differentiates it between other WRPGs.
Perhaps they could do that other RPG they were going to work on before Microsoft wanted them to do Fable? Do we even know anything about that though?
I would also like The Initiative to switch between two franchises, but who knows if they even will?