Since I briefly mentioned it in one of the welcome threads, I'll post a basic summary of it here.
-Game type: 2D platformer.
-Difficulty: In line with the whole Sonic/Mega Man fusion, the ideal difficulty would be as tough as the Mega Man Zero series.
-Gameplay: As more enemies are defeated, the hammer gains experience. With each level up, Amy learns new attacks such as 3 hit combos , a hammer assisted spin dash, an anti-air rising attack (up+attack while on ground, can be done while running), a bounce ball like attack (down+ attack while air borne), and a boomerang like hammer toss (down +attack while on ground and stationary. Scattered across the levels are upgrades that unlock new abilties such as the light dash (mapped to its own button) and wall jump. (Bring the cool stuff from 3D games into a 2D one). With proper timing, it becomes possible to string together advanced combos using your attacks. (aerial strike--> three hit ground combo--> rising smash --> bounce ball for example)
Of course to please more people, you would be able to play as Sonic, Tails, Knuckles,and Shadow in their own side stories.
http://www.youtube.com/watch?v=ntLk_FURQJgThis link provides gameplay footage of the Zero inspired combat. (A Mega Man title can move pretty fast if you know what you're doing.) Imagine this fused with Sonic style speed and that describes my idea in a nut shell.