Hope the next game is a direct sequel to Peace Walker.
Personally I'd like it to take place over 2 inter twining story lines. One where Naked Snake is young, before the events of MGS3 and he's still learning gooseberry fool (could be reminiscing about The Boss and gooseberry fool, as he likes to do) and the other story line to be set after Peace Walker.
Could be good, everyone knows how awesome and bad ass Naked Snake is but it'd be cool to see it compared to when he was a rookie. It'd make him even more badass, if he was like noob when he was young.
Although it might be hard, MGS3 is the story line that shaped everything about Big Boss so seeing him as a rookie might not have much of an impact. You know, 'cause nothing as huge, life changing could happen to him as in MGS3.
I dunno, maybe it could work. Maybe it'd be really cool. Maybe it'd be gooseberry fool. I dunno, I'm just rambling I think.
The next MGS definitely needs Big Boss though, not Solid Snake.