Unfortunately, there isn't enough space in the ROM for additional characters, and I can't enable bank switching due to some technical limitations.
Viewing post in Final Fight MD (Mega Drive) comments
If it helps, my Flatmap64 carts support 64 megabits of flat memory (no bankswitching!). They work on real hardware and the ROMs also work on Blastem! emulator (and most likely also others like Genesis Plus GX). They allow breaking the 32 megabit barrier by using a trick to move the SegaCD mapping when the CPU is accessing the upper 32 megabits, to avoid collisions.
And the cart and programmer are open source software and hardware.