I'm fine with sword and board being 2 AP. It gives one-handed with no off-hand item a niche of versatility, though both one-handed with no offhand and sword and board need buffs to compete with two handed or DW.
Single-handed could have unique skills. Maybe stuff like "Duelist" where you counterattack, or grapple abilities. Stuff where you use your free hand to do things that would be impossible with both hands occupied, or abilities that require a great deal of accuracy and focus. Maybe it could just grant a slight accuracy and/or damage bonus (perhaps just for abilities), not so much to equal two-handed or DW, but to make them not totally suck when you use abilities.
