MM8AQU - Aqua ManAlthough Aqua Man does have some pretty waterfalls (some of them even underwater?) it has a lot of flat wall textures and somewhat boring visuals. Most of this stems from the outside being mainly composed of rock textures, and the inside being mainly composed of green rocks with the blue Xs.
Possible SuggestionsAdd animated Shelln (http://megaman.wikia.com/wiki/Shelln) or Ammoner (http://megaman.wikia.com/wiki/Ammoner) to cliffs or the inside areas to give something pretty to look at.
Use different wall textures for the inside, there were a lot of blue textures I spotted at the start of the stage that went unused in MM8BDM.
This isn't even a visual thing, but it'd be nice if you could swim in Aqua Man.
And an idea for Aqua Man stage: You know that area where there's a waterfall visible that looks just like the one you fight Gorone (http://megaman.wikia.com/wiki/Gorone) on in MM8? I think that Gorone (http://megaman.wikia.com/wiki/Gorone) should be there, because otherwise, it's just a pointless inaccessible stage area.
MM1BOM - Bomb ManNothing seems to move in Bomb Man's stage at all, it's all grey with some sky. The bomb towers do look quite nice, but that's more of a skybox extra than part of the stage.
Possible SuggestionsAdd Bombomb (http://megaman.wikia.com/wiki/Bombomb)s, Flying Shell (http://megaman.wikia.com/wiki/Flying_Shell)s or Killer Bullet (http://megaman.wikia.com/wiki/Killer_Bullet)s to the outside of the stage so that the whole thing feels more lively.
Nothing else I can suggest, Mega Man 1 didn't have a great texture selection.
MM2BUB - Bubble ManFor Bubble Man stage: Outside the area, there should be some enemies like some M-445 (http://megaman.wikia.com/wiki/CWU-01P#M-445)s, or Anko (http://megaman.wikia.com/wiki/Anko)s that could swim around and occasionally spit out Shrink (http://megaman.wikia.com/wiki/Shrink)s.
MM8CLO - Clown ManAnd for Clown Man stage: Those Robots that carry hammers and whack the bell. Their "Sound activated platforms" are there, you can hear them, but they aren't visible for some reason.
MM5DAR - Dark Man CastleThe stage is undeniably epic and the textures are used to great effect, but I felt that the castle was empty and well, uninhabited.
Possible SuggestionsAdd recoloured Sumatran (http://megaman.wikia.com/wiki/Sumatran) to guard the outer walls.
Add Rembakun (http://megaman.wikia.com/wiki/Rembakun) and/or Tondeall (http://megaman.wikia.com/wiki/Tondeall) to patrol the skies.
Perhaps some Metal Cannon (http://megaman.wikia.com/wiki/Metall_Cannon#Metall_Cannon)s to make the fortress look guarded?
Add some MUSCLE (http://megaman.wikia.com/wiki/Power_Muscler)!!
Rename the stage "Proto Man's Castle" ;)
MM4DRI - Drill ManThis one isn't a huge problem because the stage is so tiny, but nothing really moves in it except for all the player-made EXPLOSION!
Possible SuggestionsAdd the falling rocks (http://megaman.wikia.com/wiki/Boulder) to the outside of the stage similar to Crystal Man's crystals.
Add some Battonton (http://megaman.wikia.com/wiki/Batton#Battonton)s, like in Wood or Shade.
MM6FLA - Flame ManApart from the oil, nothing in this stage is particularly noteworthy.
Possible SuggestionsAdd Fire Tellies (http://megaman.wikia.com/wiki/Telly#Fire_Telly) to the outside of the windows, like Heat Man and Ivory's Knee Deep in Oil
No room for anything else
MM8FRO - Frost ManThis stage gives me the creeps, as if it's part of a city that's been completely frozen over. Nothing moves!
Possible SuggestionsAdd Big Tellies (http://megaman.wikia.com/wiki/Telly#Big_Telly) to the outside of the stage.
Put some Rabbiton (http://megaman.wikia.com/wiki/Rabbiton)s in the jet sections like the Gockroaches in Junk Man, but don't let them get in the way!
Make it snow!
MM3HAR - Hard ManAnother small stage like Flame Man, this one has nice textures but it's incredibly static.
Possible SuggestionsAdd Hammer Joe (http://megaman.wikia.com/wiki/Sniper_Joe#Hammer_Joe)s to the ledges similar to the Picket Men in Guts Man.
ADD THE BEES (http://megaman.wikia.com/wiki/Have_%22Su%22_Bee)!
MM6KNI - Knight ManFor the 'Capital of Science' this castle sure is deserted! Similar to Dark Man, I'd suggest adding some minions to give the stage a lived in feel.
Possible SuggestionsAdd the Brown Barrels (http://megaman.wikia.com/wiki/Brown) to sections of the stage, perhaps the robots could pop out if shot?
Put some Brain Break (http://megaman.wikia.com/wiki/Brain_Break)s on the outside walls, the place is a Capital of Science after all!
Add the spiked ceiling as an extra, although it shouldn't be a trap!
MM3MAG - Magnet ManPossible SuggestionsDo an Ivory and use Mag Flies (http://megaman.wikia.com/wiki/Mag_Fly)!
MM2MET - Metal ManMetal Man should have the Presses (http://megaman.wikia.com/wiki/Press#Press) and Blocky (http://megaman.wikia.com/wiki/Blocky) in a background area.
MM6MRX - Mr. X FortressAnother castle with no inhabitants!
Possible SuggestionsCannopeller (http://megaman.wikia.com/wiki/Cannopeller)s and Hotchkiss'n (http://megaman.wikia.com/wiki/Hotchkiss%27n)s outside!
Leave the X Crusher (http://megaman.wikia.com/wiki/X_Crusher#X_Crusher) hanging from the ceiling somewhere?
MM4RIN - Ring ManSpace sure is quiet...
Possible SuggestionsAdd the Kabatoncue (http://megaman.wikia.com/wiki/Kabatoncue) (Hippo Platter) and Ring Ring (http://megaman.wikia.com/wiki/Ring_Ring)s to the outside of the stage!
MM8SEA - Search ManThis jungle doesn't seem as lively as say, Plant Man...
Possible SuggestionsAdd varieties of Gori-Three (http://megaman.wikia.com/wiki/Gori-Three) to the borders of the stage.
Replace the pits by the E-Tank with the electrified thorns (http://megaman.wikia.com/wiki/Thorn_Vine).
MM5STA - Star ManThe only moving prop on this space station isn't even from the Mega Man series!
Possible SuggestionsAsteroid Showers! (http://megaman.wikia.com/wiki/Asteroid) (and I don't mean Astro Crush!)
B Bitter (http://megaman.wikia.com/wiki/B_Bitter)s to make the station look defended.
MM7DW1 - Skull Fortress 1The Cyorowns are a nice feature, but they weren't even in the original!
Possible SuggestionsReplace the crows with Frisk Cannon (http://megaman.wikia.com/wiki/Frisk_Cannon)s.
Add more to the Metal Blade area so that it isn't pure black. Perhaps the large generator from MM7DW2, and you could see between stages from there? Similar to Duo. Possibly. Just a suggestion.