Well, massive enemies would be cool. As in massive enough to be the levels.
This is about the only idea that I have, and it's a little too similar to GOWIII...
Should I approach the last question in the same way I'd approach "What would you do for a Klondike Bar"? If so, here's my answer: I'd be willing to buy it. Yeah, I suck. --- With my freeze ray I will stop the world. -Dr. Horrible "Riki-Oh: The Story of Ricky" -- http://video.google.com/videoplay?docid=417949348888674336
You know what Sony could do for the next installment? make something based on Robbit Mon Dieu.
No, I am neither insane nor joking. I seriously think it could work. The original RMD had a whole lot of ideas in it that were half baked and could have been expanded much further. One thing I can think of off the top of my head is the monetary system. Being able to spend points you earned on items was a novel idea, but the only items you could buy were lame special weapons and useless candy bars with collector cards in them. Not only that, but they were so cheap and unimportant as to render your purchase almost completely meaningless. if the collector cards were ditched, and you had some cool things to spend your hard earned points on, like weapon upgrades, temporary extra height boosts and other temporary and permanent mobility upgrades, passage into secret levels, maybe doodads to put into levels you could make in a hidden level editor, etc.
There could also be more specialized levels for certain types of missions, and more differences in level design for each mission. Robbit Mon Dieu often put you in small, featureless, and sometimes claustrophobic levels that usually weren't very interesting to play. More specialized levels would allow bigger, better designed, more interesting, and overall more fun missions to take place.
In-level character interaction would also add some nice depth to the game. You could go around the levels, tell people things you heard from other people, and do various other things which would affect what kinds of missions would occur. If there wasn't currently a mission in the place you were visiting, you could explore it in a type of "town mode", where you could talk to people, play various minigames, buy things from stores, and stuff like that.
If this is beginning to sound like an RPG, Robbit Mon Dieu was originally designed to be an action-RPG with more arcade style gameplay. What do you think of my suggestions?