I'd like a grand storyline, but not too grand: overzealous plots have ruined games, like Sonic 2006.
Sonic 06 was ruined by mega glitches.
[video=youtube;3MWEHwvcnyM]http://www.youtube.com/watch?v=3MWEHwvcnyM[/video]
It's the only game you can glitch without having to hunt.
As for the plots, they should depart from the usual formula, and not try to wrap the whole game around a terrible plot. I didn't like FFX for that reason, neither did I like BW for that same reason. BW not only had the same "Evil Team Will Destroy World" plot as RSE/DPP, but tried to tie the game in with the plot more...which failed due to the fail plot.
If they had a decent plot (PMD, something else), then I would like it. But it's not worth tying the game around a bad plot, if I want lots of cutscenes and linearness, it better have a good plot so it isn't just an a button masher.