You might want to consider two variants: one that is on the KISS path, and is on the lower price knee, and just sufficent to operate a workable JOP, and another that is more maximal, and more able to launch an operating system as well.
For package, I'd vote against pin-modules, as they are expensive to make, have a pin count ceiling, and are easily damaged. The only benefit is they can save a little time in first deployment. Better are the memory-card formats, as they have low cost and sturdy connections, that can still be socket managed on 1 & 2 layer PCB, and you can offer a socket-pcb for users starting from nothing.
Besides the simmmstick itself, you should consider the SODIMM modules, as seen in TINI :
For "smart expansion", SD would be one option, but smaller and more flexible might be USB-Drive ?. Not as cheap at the lowest node, but universally portable - few PCs can R/W SD cards, but they can all R/W a USB drive. Of course, that means the FPGA needs to be smarter :)
-jg