You have a lot of possibilities! Just looking at
Skyscanner gives you an indication of the places you can get to on LCC from FRA.
Personally I'd suggest Munich - easy to get to from Frankfurt by car, coach or train, but a world apart from the other places you say you have visited in Germany - it is Baveria after all!
Or maybe take the train to Brussels, Amsterdam or Prague - all good destinations for a short break.