Metroid Prime would need significant changes to it's playstyle to be an attractive multiplayer franchise, and those changes would alienate a large portion, if not most of, the existing metroid players.

A seperate spin-off set in the Metroid universe would fare FAR better with both the existing community and those new to the series.

I have to disagree with this statement, as nobody knows how MP3's style will be compared to its predecessors.  MP3 has the potential for twitch battles as frantic as halo given the input.  MP1 and 2 were slower paced because the control setup they had wasn't designed and didn't allow for twitch gamer battles.

Naznatips, I also take umbrage with your insinuation that the reason why the later games sold less was because of added multiplayer, when there are plenty of other much better reasons.  Like how MP2 is almost the exact same game/engine as MP1.  How the game looks and plays the same.  Maybe people had already tired of that style.  I know I had, which is why I hadn't picked up MP2 until the announcement of MP3.

 And perhaps MPH didn't do so well because of the 5 minute hand cramp in induced, or the god-aweful look of all the texturing in that game, or the fact that you couldn't recognize a damn thing at any significant distance in that game.

 There are plenty of things wrong with MPH, but adding or building for multiplayer isn't one of them.  Likewise, there's plenty of good reason MP2 didn't sell as well, but I really don't think the inclusion of multiplayer is one of them.