I like the idea of light bombers. They would be very dangerous if they were more manuverable than a VHF. Although, aren't some of the Vanilla Heavy Fighter models already used as VHFs? I don't like the idea of having 3 different ships that look the same flying around...
I always think of the MK.II Heavy Fighters being a balance between their MK.II LF and VHF counterparts rather than a different class of ship altogether though...