I’d like to see armoured trains for use in conjunction with rail transport rules, and possibly a big siege gun which is also moved by rail and can bombard adjacent territories.
Armoured trains would have no attack value (units cannot rail into combat) but would defend at 1/2? and confer defence against interdiction of rail movement. Each piece would have a transport capacity and would stop when unloading the last cargo, thus representing a certain limit on safe rail travel.
Heavy tanks: KV1, Tiger, Pershing; also SPGs: fight at 1-4?
Most of all I’d go for enough units to represent all plausible techs, as I firmly believe that techs should be separate units newly built rather than magic upgrades.
This includes standardising aircraft as:
single engine fighter - standard
twin engine fighter - long range
twin engine bomber - standard
four engine bomber - long range/heavy