Flights generally still qualify for roundtrip fares as long as they meet the following requirement in fare rules --

"MILEAGE OF THE OPEN SEGMENT MUST BE EQUAL/LESS THAN MILEAGE OF THE SHORTEST FLOWN FARE COMPONENT"

While you can use multiple cities or "all airports" codes (like NYC and CHI) as origin in Google Flights, it will only show return options back to outbound city once you have selected the outbound flight.
Most DL fares out of FLL and PBI to other major metros are all one-way fares (DL hub hostage airports can be an exception). Cheapest DL one-way fares on PBI-DTW are $218.60, while roundtrip fares on PBI-DTW start at $168.60 each way. Otherwise for domestic major metros, you can generally just do one-way searches in each direction with multiple cities in origin or destination and still get the lowest fares when dealing with south Florida airports.