If you do this also figure out how to assign hybrid status.
Perhaps so the admin could mark character DruidX who is geared well for tanking and for healing (but not DPS) with a hybrid status of H.
The signup would look like this:
Tank
DruidX (H)
One way to get around that in the current system would be to make Tanking, HEaling, Melee DPS, Ranged DPS, etc all secondary roles - since at the sign-up level, you can select more than one of those at a time
the user experience would be a bit strange though