WhiteHawk,
The Do335 was never used. I wouldn't have a problem with it being added, but given its armament and speed I'd say a price of about 80.
No Japanese aircraft used by the Japanese in WWII could break 400mph sustained while level. What the Japanese really, really need is the Ki-84-I-Ko. The best Japanese fighter to see service in WWII, and also the fastest.
Why perk the Bf109K-4? Sure, add it, but it doesn't warrant being cursed with a perk tag. It'll only be slightly better than the Bf109G-10.
The P-61A or B Black Widow would be neat and some people (Ripsnort) have asked for it for years, but its top speed of only 360mph would hamper it severely. Also the changes required to simulate night fighting would be huge. Currently all we have is the MA, but darker.
B17Skull12,
A new perk bomber would be nice, but there are very few candidates. I would say that the Mosquito B.Mk XVI would be the one to do if a quick new perk bomber is desired. The B-29A would be the other good perk bomber addition, but would take HTC a lot of work to do. The B-32 Dominator is the outside possibility for a perk bomber, but it hardly saw any action and would be just as much work as the B-29A.