OneWorld+StarAlliance: DUS-TPA/RSW €300 return
Departure from DUS only to TPA and RSW on BA or LH, both alliance is competing. Fares from €300 starting from 22nd August until middle of 2018.
Other destination might be available too but did not check further.
Do not know the sales period but the maximum stay is either 90 days or 3 months.
Happy hunting.