There is a BIG DIFFERENCE between being "silly" and being "funny or humorous". The first two FF films were "silly" not "humorous"....that is a big change that needs to happen. They aren't the 4 stooges, they are the Fantastic Four.
Something that I felt was not dealt with at all was solid relationships....which are adult themes....not stupid spats between each other.
Their identity being open to everyone was dealt with in a stupid way as well....Sue running from people? was just "silly". They touched on it with people writing them letters asking for help, and I'll be damned if they didn't #1. cut the scene, and #2 turn it into yet another argument between Sue and Reed....
Just small things can really turn into big problems, or scenes that are keepers....
The other BIG THEMES of course must be dealt with, but until we know a villain, it is hard to figure out what the possible changes need to be.