I would love for Underwater combat to return, yes!
Not a huge fan of the gameplay of it in MH3, but then again everything in World feels better, I can imagine Capcom making underwater much more fun. Maybe make it fast so it feels like a aerial 360 fight where you can go in every direction and make it fast. But I'm interested in seeing the MH team comes up with for underwater stuff.
But, talking about Paradigm Shifts, the one thing I want to see in this game is them revamping the multiplayer. Make it more Destiny, where you can team up with a friend and the leader launches activities for the rest of the team. If I'm in the village/hub, let me see up to 31 other hunters just walking around minding their own business and if a friend wants to join me, let them join me in the village/hub first. The Gathering Hub feels tacked on, I want all features just built into the main thing. I think the game that does this well is Destiny, Despite all the other issues that game has, it does coop well.
They could also remove timers. and also maaaaybe the lives systems, where if your wipe 3 times you are out. I don't mind them so much, but they feel like they don't fit with the game, and the only reason they brought them is because they are staples of the franchise since the beginning. But I think MHW was able to grab the essence of a MH game without re-doing everything the same. They could change some things around more to make it its own thing.