I believe one Metroid game like the original Prime per generation (or about once every five years) would be fine.
I would not want the multiplayer option cramed in, however. While I see the potential, I believe that Nintendo should spin of the Hunter game into its own series and make that Nintendo's FPS.
I would also like to see a 2-D metroid game as well.
However, there is potential for metroid oversaturation, but make sure every one plays differently and only release one per generation and I think it will be alright (ie, like one 3-D mario, one 2-D mario, one Mario Kart, ect. seems to work just fine due to the gameplay being different). Now that I think about it, perhaps a new franchise for Nintendo's big FPS, and then only have one 3-D metroid and one 2-D metroid per generation would be better.
To be honest, however, I am worried about Metroid saturation this generation between Corruption, Trilogy, and Other M all coming out (sorry, but I am pessesmistic about Other M delivering the goods. I hope I am wrong, but metroid games to me were always about exploring the unknown isolated from everyone else. I have never cared about Samus herself, she was just my 'avatar' to the gameworld).
