Dude.. FPGA Replay is a re-implementation so any drivers are already written..
As for extensions, another FPGA would enable even more cool stuff 
If you are referring to USB, there are separate device drivers for Subway and Deneb that I know of, but I would imagine that a new device driver will need to be written for the daughterboard of the Replay to implement the 3 USB ports on it so they can use Posieden and/or Annais (spelling?). Unless either the Deneb or Subway driver would work for the USB ports of the Replay daughterboard, but I doubt that as they are commercial products and aren't likely to give away their device drivers.
As for your other comments about the possibility of future expansions, or revisions of the Replay board, or it's daughterboards, that is most likely a certainty, that there will be more improvements and additional, or upgraded expansion capabilities. The only limit is MikeJ's, or Jakub's imagination.
