MM1 - Make Hyper Bombs explode on contact, and try to keep Bond Man and whiever else in the game.
MM2 - Make it harder; it's the easiest in the series! Also, make the Wily energy balls dodgeable and scrap the Buobeam Trap. Make Quick Man have a pattern, and make some of Air Man's patterns dodgeable. Finally, make Metal Man harder or nerf the Metal Blades! >:O
MM3 - Don't give so many E-Tanks and items in Wily stages, give Snake Man, Needle Harries and some others a smaller hitbox, give it a proper intro, make the Doc Robots less durable, and I'm too lazy to list everything else I'd change in 3. :U
MM4 - Nothing.

MM5 - Make Wave Man's stage have proper water in it, and make more Gravity switching. :/
MM6 - Should've had some stages with gimmicks like with the spike platform in Tomahawk Man's stage, and a stage dedicated to the Rush Adaptors.
MM7 - Nothing, it's my favorite in the Classic series.

MM8 -
MM&B - Balance out the difficulty, please. :/
MM9 - Easy mode.
MM10 - Maybe make it not 8-bit.