For me, 64MB in an A1200 is a much unwanted downgrade, my aging A1200 already has 192MB and for the work the Amiga is actually usable for - simple 2D animation - 192MB is limiting. In my A4000 I can fill up the zorro bus with 1GB of RAM, but then I have no space for other cards. Only viable option for legacy software is really emulation.
In my view, to put 2GB on the card is well worth it. I would make a card that is essencially an FPGA computer, with a lot of RAM, some flash and I/O, and use that as base for all Amiga computers with different connection board for the various models.
64bit memory addressing for m68k has no use unless you have more RAM anyways.