Metroid Prime was great. MP2 left me unimpressed. MP3 again had some great new ideas and was awesome. But even better than the Prime series are the 2d titles. In Fusion you were always told where to go, which took from the exploration-gameplay, but therefore the puzzles were more difficult, and so were the bosses. Zero Mission is very similar to SM, but it's like comparing AoS with SotN. SM had great atmosphere and revolutionary leveldesign, whereas ZM was technically very good and allowed fast, skillfull gameplay. You see, though they are all 2d sidescrolling Metroid games, each one is unique.
So, yeah, maybe they can go with Castlevania like Nintendo did with Metroid. I wouldn't mind.