I would like to see a return to RE 2's style where you have two main characters each with their own story. The LeonA/ClaireB and ClaireA/LeonB thing was brilliant. Don't know why Capcom never reused that concept. They somewhat did with RE 6 but it wasn't nearly as effective as they spread themselves too thin I think with four main campaigns.
You were the lightning in that rain. You can still shine through the darkness.
I'd like to see a single campaign that you can play through as Character A (ie solo) or as Characters B and C (ie co-op). That way people who want the old-school single-player experience can have it while co-op people get theirs too. The paths would vary sligthly but it's more of a choose-who-to-send-in rather than characters doing their thing at the same time.