MP1 is awesome and a desperate cry for developers to remember the art of level design. "THIS IS HOW IT IS DONE, SEE?"
Echoes unfortunately used the level design theory of Fusion, which is a flowchart like design which has a head/top, from which areas bracket into below, but it is less creative and in the end doesn't work as well. On the other hand, the morphball puzzles were improved and the bosses were great. The fortress area(forget the name) was a new look for the series that worked well as a seperate entity.
The worry with Corruption is that they have been listening too long to the haters, that really have no interest in the game, and have turned it into a generic FPS. Early reports say things like "Halo like" and discuss less twisting level design and redundant pathways. What makes Metroid Prime or any Metroid great is that it perfects all the elements of game design, be it control, level design, bosses, puzzles, music, atmosphere or pacing. You need a genre that relies on those things, so it was always an adventure game with action.