If it can be battery-backed and bootable on demand, that would make GEOS pretty nice. Other than that, I would prefer a SuperCPU-like expansion.
There already is a SuperCPU like expansion made by Jens with REU, SD-1541 emulation, VGA output and MUCH more..... I have one and they work very well. It is the Turbo Chameleon.
This is for people that dont want/afford the Turbo Chameleon and want a REU like the CBM units but maxed with out RAM. If it is the same REU code used in the Chameleon then it should work very well.
One of the nice things about the REU for the C64 was the DMA engine the REU had to move data in and out of the REU VERY fast... Almost like a blitter in some ways. Of course GEOS and other items make good use of it as well.