Many good suggestions here, but I think the most overall sought after, and thus most realistic products are:
1. USB cards. If it isn't possible to produce more Deneb and Subway cards, then at least something similar for zorro slots and clockports. Maybe even a PCMCIA to USB adapter for the A600 and A1200.
2. Zorro SATA card with maybe a couple of clockports as well.
3. Tower cases for A1200. This shouldn't really be that difficult or expensive to make since all that's required is standard PC cases that are modified to fit A1200 motherboards. The cases should of course be compatible with Mediator busboards.