Anyone who hasn't yet should try Zero Mission, preferably on a DS or other non-GBA system, as the GBA has a poor screen compared to the alternatives (DS, DS lite, Gamecube too, possibly?). It has a copy of the original (was it an unlockable thing, or was it always available?) slightly altered to be able to remember one password for you, and it has eight distinct end images that can be unlocked. Besides that, the game itself is very similar to fusion(with the addition of infinite waljumps on a single wall), only in a *very* freeform world with enough hidden passages, secret areas, and workarounds that you can complete the game with less than 10 items, and likely will accidentally use some of the shortucts on your first play through
without even realizing it.
Also, ridley on hard mode with minimal items: Very Hard. One hit death from one attack, most/all others can kill in two, and since ridley moves a lot... It would fit very well in I wanna be the guy.
Fusion has a rather difficult secret and an unfortunate lack of sequence breaking.
The secret involves attempted sequence breaking through a very difficult shinespark course, and all you get is a message. Overall, the reward is finally getting there, not what you find there.