The sky is the limit if you want to build something. I had been thinking
of something like that too, and soon couldn't stop adding features. The FX send from your preamp could go to a buffer stage to fan out out to any number of effects. On the return side, a mixer circuit for any combination of
stereo or mono outputs. The buffer and mixer circuits could be a single tube held in the same box as the footswitches used to select your effects, as well as simple options for A-B-Y. The circuits are all basic building blocks, and you can get as fancy as you want. Why don't you post you skill level, and maybe others can contribute some circuit ideas.
Otherwise, get out you checkbook and get yourself a Carl Martin, or any of several commercial switching units.