That is, of course, assuming you're not allowed to design a completely new power.
NOTE: The improvement doesn't necessarily have to be on a crappy weapon. Maybe there's a weapon that you already like and would like it even better with some tweaks!
MM1Super Arm — Creates blocks out of thin air with 4 ammo; blocks that are part of the background don't spend any energy
MM2Leaf Shield — May walk with shield on; if you throw the shield UP, 4 leaves float down à la Wood Man (costs extra energy)
MM3Top Spin — Pulls all on-screen items, like the Charge Beam in Metroid Prime; weird hitbox fixed
MM4Skull Barrier — I wouldn't make it Jewel Satellite broken, but it would at least block normal projectiles without disappearing. Also, since this weapon is Dive Man's weakness, it would have some kind of unique underwater effect, like increasing Mega Man's speed
MM5Power Stone — The cluster would be lagtastic, but there's no way you wouldn't hit
something with 2 casts on screen
MM6Plant Barrier — Bounce like the Plant Man springs while using it? I dunno, it's hard to think of a shield improvement that isn't "be Jewel Satellite"
MM7Freeze Cracker — "The Freeze Cracker fires ice bullets, doesn't it?" "Yes, but it is freaking SLOW. Here, let me increase the speed and rate of fire for you."
MM8Thunder Claw — Allow swinging on any ceiling
MM&BMagic Card — These cards would be more magic if you could throw them diagonally
MM9I'm interested in reading your input. I can't think of any improvements, all weapons here are very powerful and useful to me
MM10Thunder Wool — Cloud phase would be completely
in-des-tructible (I do not recommend clicking that link)