Multiplayer: vehicles could help, the revmote will surly help, and any mention of spring ball is good news.
I can sympathize with the people who felt Prime 2 too dificult, but with the revmote things will go smoother. I wouldn't be upset if part 3 was easier to get into; what I want though is a greater sense of AI (or lack there of) in characters like in Halo.
Samus I feel is more a bounty hunter than a starfighter, but vehicular options during the game can be fun; once again, I point to Halo as evidence. But if the vehicular sequences were like say, StarFox Adventures, count me out. I'd rather think of the vehicle in gameplay the same way it is thought of in GTA or a horse in Zelda. Back to bounty hunting; it would be interesting to do some actual bounty hunting, and I would figure Samus might have a few hunters after bounties on her head.
The third person view in my opinion should still offer up all the controls of first person (though auto aim would likely get used more in third person), but the revmote could be toggled to allow you to do special melee and stealth moves when in third person. A sword would be nice in the game. Jumping would be easier in third person. Samus definately needs to be faster on foot (the revmote should solve turning speeds) and a rocketpack upgrade near the end of the game could be fun. Or you could just snatch them from dead enemies like you take an enemies gun in Halo. The catch would be you eventually run out of fuel, the pack slows you down when on foot, and you can't jump so high when carrying the pack.
"Potentially cool abilities like the grapple beam are wasted because they require predetermined situations to be used at all."
I think the grapple beam should either be included into Samus' arm canon arsenal or should be a type of electric gauntlet you attain; anyways, it should be used anytime you want. One should be able to grab enemies and throw them, pull in power ups, and generally grab any environment and swing from it. I could see players grabbing pirates and beating other pirates over and over with the dead pirate's body from a distance using the remote. Or you could torture an enemy shocking them repeatedly until they give up information.
Edit: I tried to be short but since I failed I'm going to share some of my ideas for the next Prime. I want to use the mic to not only talk to in game characters and to other players (coop or team deathmatch), I want an echo canon that generates a beam according to the sound you create. The pitch, volume, key, note, rhythm, etc. will affect color, shape, size, strength, accuracy, reach, etc. of the beam.
Stealth cloaks specific to the visor technologies of the particular enemy you are facing. This would play out best in death match where pirate, marine, and other characters other than Samus will feature different hubs with unique visor set ups. So maybe you're invisible to the naked eye, but your pirate buddies have heat vision available. Or you think you are safe in the dark, the only one who can see, but behold the other bounty hunter has green night vision. There definately need to be more alien creatures with unique technologies in the next Prime. When playing through the first prime I kept thinking about what the pirate saw, because there was such an emphasis on Samus' hub and visors. It all made me think of Predator.
Also, what about remote controlling the rockets, even sticking them in enemies, and blowing them up when you feel like it (like when they run to their friends for help).
And I think it would look cool to have a beam with a rotating barrel.