That would require a substantial change the system design... there's NO space in a mini for any sort of removable graphics subsystem, so it has to be part of the mainboard. To have different GPU options means that they'd need to have a separate logic board for each option, which just isn't practical. If you look at all the other Macs, the GPU options "make sense" from that point of view.
The 17/20" iMacs have the same video card (which is part of the logic borad layout) but different RAM configuration, meaning the board/firmware is the same but there are just additional chips soldered in. The 24" iMac has a removable GPU unit, so it's possible to offer other chips. Same goes for the Mac Pro.
The best we could hope for is a mini with the MXM notebook GPU card system, but that's quite expensive and so very, very unlikely. After that, they COULD put in a discreet mobile GPU like they did with the G4 minis, but since it looks likely that the x3000 will provide similar performance for less price it's much more likely they'll just do that.
I seriously doubt the x3000 is anywhere near a match for the Go 7600 mxm II card which fits in many a 14" notebook