I'm just thinking up vague ideas for future perk planes. The idea I've got lately is to think up a list of planes that add variety (something to spend perks on) so that once you earn perks you can use them on something. These planes would ALL be perked, and not necessarily at cheap prices, depending on the impact on gameplay (the impact on gameplay would determine how many perks, but all would be perked for rarity reasons). The point is to add perk planes that are NOT perked because they are super planes, nor because they are uber late war planes, etc. Perked for scarcity/rarity.
a couple I suggested start the list out just because I'm thinking as I type:
He100D (not likely)
Fw187
G.55 (might have to perk the c205 for rarity in this case, too)
SpitV with 4x20mm (Yeah.. I have to add this but I think it'd have a high price)
F6F-5N with 20mm (I doubt it but I'll add it to be fair)
Fw190 with a torp under it
Soviet C-47 with dorsal turret and 1xShVak (I think)
190a5 with a 2x20mm gunpod under each wing (4 in pods 2 in wings 6x20mm total but loses lots of performance) -- this was used but relatively rare.
B-17 with heavy payload (B17s could carry over 10k, but operationally took 5-6k. It was rare, but could do it for shorter ranges, so perk it)
Bolton Paul Defiant (pretty useless, IMO, but kinda rare)
Brewster Buffalo (rare as hell and flies like crap

)
Add other solutions here. If it totally out peforms other planes in several areas, then chances are it doesn't fit this topic.