(This is a bit like my thread “To accompany a Syntakt” over on the other side(
Honestly, The DN2 has just about everything “synth” covered, especially if you layer more than one sound. There’s a lot of things it could do more easily but not many sounds it can’t make.
The obvious next step for “more varied sound palette” is a sampler but you have to enjoy finding/creating/editing sound files for that which usually means sitting at a PC with mouse/keyboard. I think most people who spend money on hardware are trying to get away from that, so… 
One synth that really impressed me with its sonic abilities in the last couple of weeks is the Behringer Pro VS Mini. It’s a bit fiddly to program but gives huge results. It’s only $90 so it’s a bit of a no-brainer.