Oooh, I hope we'll get a big graphical leap for the new generation.
The last MAJOR graphical update the LEGO games got was LEGO Star Wars 3 back in the 360 PS3 gen. They've looked really good ever since then (LEGO is fairly simple so you don't need to be too crazy), but if they could add some great Ray tracing and material effects that would be really awesome.
As for which franchise it is, I'd love it if they did a full X-men game. Like X-men Legends 1+2, but LEGO.
Lego Marvel Cinematic Universe, all 22 movies.
That would be insane.
They've actually done quite a few of the movies in the past Marvel games (or at least "inspired by" versions and some of them were dlc levels), but having all in one game would be pretty crazy.
I imagine they'd only be able to give each movie like 1 or 2 levels each in order to fit it in one game.