Don't know what they are planning in Germany. But if they really want to grow to 20% market share they will have to do domestic flights. From my point of view it doesn't make sence to go to SXF.
MUC-DUS; MUC-TXL; DUS-TXL; CGN-TXL; HAM-MUC; MUC-CGN are the main ones and FR should go for them to compete with LH (operated by 4U) and AB.
To do so they need at first slots at MUC, DUS and TXL which might be difficult.