My P5 checklist includes... well, the opposite of what most of you want. Haha. I love the P4 cast and the entire atmosphere.
Dark is pretty often overdone (poorly) these days, but I wouldn't complain about a darker storyline, either, as long as the characters are still likeable. These guys have done dark before, and done it very successfully.
Meguro and Kitajoh on music again, of course. While I would have been happy with more music in the style of P3/4, I'm thrilled that Meguro is going to change it up again for P5. Always love these teams for changing things up so much.
And I actually really like how they tie these games together, so I can't say I'd mind seeing appearances from P3/4 characters.