Game style: 2D Platform for handhelds, 3D Platform for others.
FPS/TPS - Ratchet and Clank: Up your Arsenal has the option of having a 3rd person perspective, first person, or third person with first person controls. Either one would be good.
RPG/MMORPG: It should have a weapon-upgrading and health-upgrading system like Ratchet and Clank: Going Commando/Up your Arsenal.
RTS: Might make a fun minigame. JazzDevan Wars, anyone?
Adventure: Definately a truckload of adventure elements. ^_^
Fighting: It might be a good idea to incorporate melee attacks, just in case you run out of ammo or your Blaster is useless against the enemy.
Racing: There should definately be some footracing of hoverboarding. =D

Platform: (multiple-choice): Doesn't matter. The multiplayer feature should be free like Ratchet and Clank: Up Your Arsenal.

Time period: Sequel. I'm also thinking of a JJ 2.5 or something for comic relief or similar.

Graphics type: I think it should be a combo of cell-shaded characters with realistic backdrops, like Legend of Zelda: The Wind Waker.

Multiplayer Gametypes: (multiple choice): There's no limit, but there should be some minigames (like racing) to break it up a bit.

Other companies involved: Two words: Insomiac Games. These guys would love working on such a project, as long as they get the rights from Epic.

Cinematic scenes: Depends. I've got a storyline in the works, but there hould be just enough substance and humor (Ratchet and Clank) to keep things interesting.

Nintendo DS: I've been thinking...Why not have the weapons and data on one screen while your character runs around the uncluttered main screen? THAT would be cool.