I remember the days when I used "Construction Kits" on my Commodore 64 to make games and share them with friends. Nowadays, I think it would be fun and easy to create a "boss fight" monster to try throw at your friends. If this game was a small project, it would be a 2-D "Smash TV" like battle with power-ups for the player(s) while the boss can be as big or small as you want. If this was a big project, it could be "Shadow of the Colossus" in scope with 3-D scaling. Either way, you should be able to select body styles, appendages, and the ability to take different forms (after a certain amount of damage, etc). Finally, the game should help people in making a good boss fight by recommending how many damage points, damage zones, and other tips so one does not create a freakishly long and boring battle.