It's never possible to have too much return air. Your furnace will only pull what it needs to in order to make up the air for the supply. You can add a return to the unfinished side, and it will help supply the furnace with an ample supply of air, but it will do almost nothing to help the finished side. Heating vents need help getting the air into the room. The return is that help. The return actually "pulls" the conditioned air into the room. With no return, the room pressurizes and no more can be supplied. Add a return, and the room balances out.