I've tried to figure out a way to make a great crossbow actually usable but i can't do it.
I read now on the wiki that the deepwood sniper shot works on all 3 of the shots from a repeating crossbow. That would have been probably the only thing that might have given advantage to a great crossbow, due to it's higher threat range. Now, there's nothing. Sure, the Great crossbow is a 2d8 weapon. A greensteel one would even be 3d8. Since a repeater fires 3 shots though, it's essentially the same thing, and anything that increased damage on a crossbow basically gives 3 times the improvement to a repeater as to a great.
So i suggest changing the multiplier from x2 to x3. Make it a critical hitting monster. Especially when used in conjunction with the sniper shot.
Maybe make, i don't know, one named great crossbow in the game? That one from Korthos doesn't count.
I'm sure almost nobody actually cares about great crossbows, but i think it would be nice to make them viable at least for a flavor type build.