Two tricks to extend the value of a Delta award ticket, when you really only have the one-way need:

1. Book the return off in the far future. You can "bring it forward" later when you might have a reason to take that return flight.

2. Use Delta's liberal "open jaw" provision to get the return leg from a city you are more likely to come back from, and proceed as in 1. above. For example, travel one way SFO-ATL because you need to now. Book the return New York or Miami or wherever else back to SFO, based on a trip you might take over the next 12 months. You might choose this "future' city based on markets where a one way ticket can be cheaply purchased to complete that round trip.

By the way, in AirTran's current program, you do in fact receive two free one way awards when you cash in the 16 credit "coach round trip" award.