A Game Boy Advance core is in development for MiSTer. It is based on an open-source Zedboard FPGA implementation by a group of students from Carnegie Mellon University and is being ported to MiSTer by ElectronAsh. The core currently supports loading the BIOS and one game, a homebrew Mario Pong game created by the same group.
GBA FPGA Final Report: https://github.com/soctar/GBA/blob/master/final_report.pdf
Open Source GBA FPGA Core: https://github.com/soctar/GBA
Mario Pong: https://github.com/soctar/GBA/tree/master/games/pong
ElectronAsh’s MiSTer Fork: https://github.com/ElectronAsh/GBA