I've been thinking of a hack for TMNT 2 on the NES. Basically you'd be playing as Bebop and / or Rocksteady instead of the turtles. Could be called TMNT 2: Bebop and Rocksteady's revenge. I even came up with a little plot for it
Basically Shredder has fired Bebop and Rocksteady due to their constant incompetence. Instead of accepting their fate, Bebop and Rocksteady decide to foil Shredder's latest plans to prove their worth to him, before the turtles can even arrive on the scene.
So basically replace the playable turtle characters with Bebop and Rocksteady. The turtles have 3 ways to attack enemies: weak attack, strong attack and jump kick. Bebop and Rocksteady would have 3 ways to attack as well:
Button A: Punch (Bebop), kick (Rocksteady). The basic melee attack, would deal same amount of damage as the turtles weak attack.
Button B: Ray gun (Bebop), machinegun (Rocksteady). This would be a new gameplay element, since it would be a long range attack. The damage should be pretty low (maybe taking 4-6 hits to kill a foot soldier?), so there is a nice balance between long range and melee combat.
Button A + B: Running bodycheck (for both). This would result in the character running half a screen (or full screen) while dealing heavy damage to anyone caught in their way. To balance it, using this attack would damage you (just like the super attacks in TMNT 3 do). It should not be possible to cancel the running attack half way, since that would allow players to spam the attack and do massive amounts of damage quickly.
Neither character would have the ability to jump with these controls. Though I can't remember jumping being absolutely required in more than 2 levels. In the sewer you need to jump to be able to get out of the water. In the skateboard level you need to jump to hit the choppers. These would need to be worked out, or simply remove these 2 levels from the game. Would also need to replace the level 1 and 2 boss fights, since fighting Bebop and Rocksteady as Bebop and Rocksteady would be kind of weird
I think this set of attacks would make the game more dynamic and interesting than the original game. In the original the weak attack doesn't have much use, usually you are just using the strong attack on foot soldiers, and jump kicks on bosses. For Bebop and Rocksteady both the melee attacks and guns would be different and usable options. The bodycheck would also be very useful on large groups of enemies, or on bosses.
I don't have any knowledge of ROM hacking so I don't know how doable this idea would be. It would require more work than just replacing graphics, since the secondary attack and super attacks would be different than what the turtles use in the original game. Here's a concept picture of what it might look to play as Bebop and Rocksteady: