At this point, I'm not even sure of what I'm interested in. So a literature survey is in order.
I've always wanted a crossbow capable of throwing bolts in the 1 to 5 pound weight category... Which I gather was not unheard of in the windlass bow era. I remember reading The Saracen Blade as a child, and I've wanted a bow too powerful to span by hand ever since.
I've build a little test-bed that I can mount to a workmate type bench, to experiment with different configurations. I've acquired some fiberglass tension-rods, torsion springs, surgical tubing, some 250lb test dyneema braid and I'm making some plywood cams and pulleys. That should let me try out different in-swinging and out-swinging configurations with variations on power-stroke, and power source... I'm probably going to have to acquire some thera-band gold, since Jorg Sprave gets some impressive results out of the stuff.
For the test-bench, since I'm going to do a scaled down model, I'm planning on throwing this little quarter-pound bolt that I turned up on my lathe:
If I set up my lathe for a bit of production, I can make a bunch of them, and I think I have a good idea for a magazine to feed them into a bull-pup arrangement.