Just going by the pattern set by the Developers of MMM we get roughly 4 RMs per game (looking at MMs 1-6), no idea if that will change in the future. More recent updates have Mega Mans 7, 8, and 9 with only 2 RMs represented.
Based on the predetermined rules established by the Developers for compatibility with MMM the following Robot Masters of 7, 8 and 9 seem most likely.. I'm not saying the other robot masters won't ever be in MMM, I'm not a developer and I don't pretend to be, I'm just going by the guidelines of what those in authority (the Developers) have laid out.
After careful review I personally chose MM8 with Frost Man and Aqua Man as my most wanted/preferred of this group.
The following are my personal preferences and opinions...
MM7
Freeze Man: My #1 Choice for this group simply because MMM could really use more Ice Themed Robot Masters. Jumps around so he's great for uneven floors, but his attack pattern uses ceilings. I suppose he could still use his ceiling icicle attack, but it not work if there isn't one (blast just flies away into the air). His floor ice blast could be made to scale uneven floors, but it's ability to freeze floors into ice would (probably?) complicate things coding wise.
Burst Man: My #2 Choice for this group. Would work with multiple types of rooms, can attack at range, doesn't require alot of special treatment.
Cloud Man: The easiest of this group to include in MMM and my least liked. Having Astro Man from MM8 already I'm not really interested in another flying orb-like Robot Master.
Slash Man: His jumping makes him ideal for MMM. Interesting melee style attack pattern. His jumping offscreen to drop red egg blobs would have to be removed/edited. Maybe have him jump into the air and hover while throwing the red blob eggs?
MM8
Frost Man: My #1 Choice for this group again because of his Ice theme, but also because of his design. Sprite design for Frost Man as well as sprites for his attack could be taken from MM10, making him roughly the size of Concrete Man or slightly larger. His ceiling attack of ice blocks could be like how MMM has MM11 Block Man bricks drop from the top of the screen.
Aqua Man: My #2 Choice for this group because of lack of water themed Robot Masters. His pound floor/splash water from below attack could be made to auto spawn water from below regardless of whether water is in his room or not, similar to the ice block suggestion above. Then again if he's in a massive arena type room how would this work being potentially far removed from the lowest floor of a room? This part of his pattern could possibly have to be cut.
I'd accept that just to get him included into MMM.His animation of drawing of water from below for his direct-able Water Cannon stream attack isn't necessary and could be left out. A Hydro Man who jumps, shoots Water Ballons, and the direct-able Water Cannon stream would be plenty for him as a MMM boss.
Tengu Man: Very versatile Robot Master since his attacks are primarily done from the air. Even though he's not my main choice his attacks would provide some interesting uses in MMM rooms, maybe even the most interesting of this group of Robot Masters.
MM9
The wonderful thing about MM9 is how every Robot Master would seem to work excellent in MMM due to their attack patterns with the exception of Splash Woman. The Devs could alter her in the same way they did Bubble Man with a water free room. They'd have to be creative about it as she doesn't use bubbles, maybe use her trident as she sits in one place?
My two preferred picks of this group are Magma Man due to his design and lack of Fire Themed Robot Masters, and Hornet Man due to his unique nature theme and design.
I'll fix it.
When a game is this good is so easy to wanna gorge yourself with all this awesomeness.
