MM1ICE - I feel like most of the fighting goes on in the upper part of the level, which leaves bottom part is underutilized. A possible solution is to level out the ground a bit underwater and remove the pillar in the center of the bottom level.
MM2BUB - I honestly like this map for the labyrinth fight, but the fear of hitting a bomb is present. Perhaps instead of at the ends of corridors, put them on the ceiling? If so, the ceiling height for each hallway would have to be increased slightly, and right next to the intersections should be some breathing room with no mines. It would also be canon with MM2.
MM3SPA - In most cases, this map feels too small and clustered. I like the tower in the middle with the 2 levels, but maybe expand the map out? (I'm thinking of a design similar to the upside-down wily level in one of the map packs)
MM5WAV - Once again, a map I like the gimmick, but I feel like the map is about 50-50. (half land, half water) What makes this bad though, is that the land is concentrated in 3 spots where the water is concentrated in 1 spot. There's not much variety going on here.
MM8SWO - One particular spot that annoys me, is the area with tango. Even though there is a tornado hold, there's not many other ways to escape. What I think could fix this, is having a thunder peg on the ledge above. Not only does this add another escape route, but it could be used to thunder claw up to the window, opening another escape route. (It might look silly however)
MM1ICE - Yes, and you know what would be cool? It would be cool that small sections in the original game could stay small sections in MM8BDM... Because there's only half a screen with the water part! I suggest to make only one big floor, with ground, water, pipes and more!
MM2BUB - Yes it's true, bombs are deadly. I suggest to put bombs on the ceiling or let them be where they are BUT adding a warning sign.
MM3SPA - I confirm that this map is too small. But would it be nice to make it more big and less complex? For a little map, all the elements are too complex.
MM5WAV - Again, what I said for MM1ICE, there's only one screen that presents the wavebike section. Something that it would be better if it could be like: a ground "donut" and water between... Or upper part with ground and bottom part with wavebike water gameplay.
MM8SWO - You know, if thunder pegs could be added to each MM8/MM&B map, people would not like them, because it would be too much easy to win... Imagine the thing: one player would have Thunder Claw, then he use it on a peg, then he's propulsed and he would use another peg, and finally pick ammo. Then repeat. Weapon, peg, other peg, ammo.