You have to incorporate this stuff into the game, as a, sort of, easter egg.
Maybe, something like old Mortal Kombat 2 that unlocks stuff.
For example, if you beat first boss only by using the legs, in the second level, enemies explodes when they die.
Or... some bonus level where you drive something (skate, or motorcycle) and you carry a rocket launcher, and shoot into enemies that explodes like this... something like Captain Commando driving bonus levels.
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P:S.
I said "you have to"... I am not demanding anything of course... just giving ideas.