Mega Man Zero 3
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Zero makes a GBA return in his third adventure on the Game Boy Advance. Along with a new weapon Zero has called the "Recoil Rod", there is something that allows you to customize Zero called the "Chip Custom". This sounds like it will function similarly to the Navi Customizer in Mega Man Battle Network 3 that allowed you to place certain programs to increase health, power, and speed along with other special abilities.
Naturally, Mega Man Zero 3 will be insanely difficult.