The GBA series has processors for both old gb/gbc games and one for newer gba games. I dont really care about waiting for GBA dev to match lsdj/nanloop (ten years wait is long enough!) I just want to make it easier to use a more powerful system with existing tools that work well.
The way the gba knows what kind of cpu to use is the shape of the cart. GB/GBC carts flip a switch in the cart slot and forces it to use the gb processor, and shifts the voltage on the link port. This link port voltage is why you cant use link cables with the goomba emulator (amongst other reasons).
Hardware development to better use the gba hardware seems much more feasible than waiting for software on a homebrew project to mature.