I've been thinking about things along these lines for a while... not that I have the resources to do it... but it's fun to speculate about. ;-) I don't think you're going to squeeze an updated graphics chipset onto the joystick or printer port. I was thinking of something that ran in parallel with the Amiga's chipset but at higher color depth. The extended graphics would be supered over the stock chipset, genlock style. Maybe the thing could work at a VGA scan rate and delace the stock chipset.
The other thing I had in mind was a "Super I/O board". There are lots of little breakout boards made for microcontrollers that do some cool stuff. TCP/IP, MP3 players, WIFI etc. Many of these seem to use the SPI bus. I figured a Zorro card that could host a bunch of these would be fun.
Anyway, it seems to me a lot of these could be implemented on the Replay.
In other words, I'm with freqmax. ;-)