I believe that the legacy gameplay fo the franchise itself limits it. I mean, Metroid Prime is an FPA. Why are we trying to pigeon-hole an FPA game with FPS multiplayer?
Even looking at Metroid Hunters, it's only half "Metroid game" and the other half straight-up deathmatch FPS on the DS. Sorry to sound like Ian, but that's the truth.
The thing is that Metroid as a franchise has never emphasized multiplayer, it has emphasized solitary exploration. It has never emphasized combat skill, but pattern recognition and exploration.
To take a Franchise that we expect that sort of gameplay from and expect to see a robust multiplayer mode that will be widely accepted is, in my opinion, extremely premature.
If anything, I believe that for Metroid to incorporate a multiplayer more, the lore has to be developed and decentralized away from the Samus character and an entirely new way of interpreting competitive or cooperative gameplay has to be invented.
How? Let me give you my 1-minute half-baked idea:
Don't make Metroid a plain deathmatch game. Make it a competitive Indiana Jones game. Make multiplayer competitive levels which are still small enough to induce frequent player collision, but also large enough to include several platform - exploration - PlayerVsEnvironment puzzles.
In such a mode all the players will start out away from each other on the peripheries of the stage. The stage will be littered with power ups, weapon and suit upgrades hidden behind challenges reminiscent of 1-player gameplay that they will need to be more effective at killing each other. In such a way, players will be encouraged to explore the still-small level and do those Metroid-esque challenges, before they want to risk open engagement in the central arena of the stage. Upon dying, they lose all those power ups and respawn.
There are two ways to win the game. Either be the first to reach a set number of kills... or upon a total number of kills being made, a path opens up which exposes rooms with 1-player-esque challenges that lead to an artifact. If you can get to the artifact and bring it to the center of the main arena BEFORE someone makes the "winning kill count," then you win.
.... Something tells me I've gone overboard.
~Carmine M. Red
Kairon@aol.com Edit: Or use the above idea but instead of counting kills, count "minor artifacts" that can be collected because they randomly spawn at various points in the level, or that are left behind when a player dies.
Edit: OR, have another style of competitive mode where the key is to get the most bounties. This would require a lot of NPC enemies in the level (meaning a centralized server probably, an additional cost I suspect Nintendo would be loathe to spring for) and the player who killed the most space pirates, through expert exploration of the level and through expert disruption of other player's efforts and avoidance of their attempts on him or herself, would win.
Edit: These ideas are dishearteningly complex, and would require big bandwidth and possible a centralized server. These are financial obstacles that I doubt Nintendo would want to risk.