I've thought about this before, and Im still tempted by it; doing a super expansion for the plus/4. One with Extra RAM, SID and MMC all built in. this would be pretty cool, to the extent that someone might even code something for it!! Having simple access to mass storage (MMC) would also let you fill the extra RAM with ease, and mean your not bound by disk loading times etc.
Still, thats a long way off happening (if ever), but it would be pretty cool I think. I wonder how many people would want one? Could be fun!