Berlin
I know/met a number of people using the BHX-HAJ service and on by train/rental car to get to the Berlin area (and I have done that before as well, outbound via HAJ, return from TXL) - or that vast sparsly populated space between Hanover and Berlin. At double-daily to HAJ far more flexibility that way.
I think TXL is one for BE - if they can make work some of these other destinations they have chose to serve recently, TXL should work out well.