The easiest solution would be to produce the power outside Germany, in places where the climatic/political conditions are more amenable to it, and then run HVDC lines.

Clean Options, alphabetical:

Advanced nuclear : Build molten salt reactors in the Czech republic, France. (these are two of the nations with the most actual, current, research into this)

Boring nuclear: Same, but with PWR's.

Renewable: Solar: Longer HVDC lines. Into north africa. Lots, and lots of solar. Mostly this gets you increased reliability and removes the "winter" problem.

Renewable: Wind: North sea floating windmills, produced shipyard style. Domestically, build http://eduard-heindl.de/energy-storage/energy-storage-system.html to level out supply.