Some sort of ranged Power Attack feat with the crossbow might be interesting. I mean, sure, you can take Rapid Reload and Rapid Shot and turn your PC into a rather strange machine-gun "archer", but that's boring. If you wanted to play a machine-gun archer, you probably wouldn't have picked a crossbow in the first place. OTOH, the one-shot kill schtick is one that's rather poorly treated in D&D (deepwood sniper notwithstanding) and one that should work well for most crossbow users.
I'm thinking +3 to damage per -1 to attack, as a full-round action. Possibly limit it to 30 feet or the first range increment or something. Hmm....
I'm thinking +3 to damage per -1 to attack, as a full-round action. Possibly limit it to 30 feet or the first range increment or something. Hmm....
