Since you refuse to do all the good concerti, my suggestion would be to do something sihtty like Hummel or Busoni. Good luck.
shostakovich no.2! at least you'll have funnot hugely difficult, im learning it at the moment, im a bit suprised youve knocked out some of the greatest concertos ever written in one fell swoop but meh whatever you fancy
If it were two years from now and I had to choose a concerto that was out of the mainstream, I'd go for like the Sauer no. 1 or something. I don't know exactly how hard it is, but it doesn't sound insanely difficult, has a gorgeous slow movement, isn't any longer than most concertos, and is played by almost no one.
Do you have the sheet music pdf for Sauer 1? I'd be curious to take a look at it.