Bf109G-6/AS or Bf109G-14/AS would be great.
Only Spitfire line addition that I think is really significant would be a Seafire L.Mk III.
Yeah my reason for the AS models is frankly we have very limited bomber interceptors that can handle the job at high Alt. Sure 109k4/G14/d9 can do the job with some speed, however what other options do we have available?
I wouldn't mind cruising around at 25k if there was some more options to intercept buffs, however given the time to intercept climb rate is an issue mainly.
Sure a P47 could do the job for example, but trying to climb to 25k gives the bomber time to run home like most do.
The G.55 comes to mind, not sure what it did around 20-25k though I assume it and the Ki-100 would be interesting at alt (although i am not making claims here just curious)