Metroid Prime 2 is the next installment in the MP series, 2 of 3 total and the 2nd for gamecube. While Metroid Prime 2 is an extremely quality game, it simply wasn't a big enough improvement over the original Metroid Prime.
The graphics showed marginal improvement. The weapon animations were slightly better, as well as the HUD displays, the reflections, and the door features. The monsters / enemies looked roughly the same, as well as behaved the same.
The selection of weapons, in many resepects, declined. Limiting the ammo of the other beam weapons was a step in the wrong direction, and reducing the overall number of weapons was also a bit depressing. The light / dark world was a quality twist, yet could be annoying at times.
The addition of multiplayer was a big plus for MP2, though the multiplayer is vastly overshadowed by the the single player mode.
All in all, this is still one of the "must-have" titles for gamecube, yet could have perhaps been better.