Its been a while since HTC added a new variant to an updated shape, IIRC the Yak-7 and 3 and the Sea Hurricane were the last. There was a big gap while AH3 was developed but since then we could have had a Ki-100 as well as the Bf-110F and the F4F-3 (assuming that's not going to be an F4F-4 hangar option). I don't think it has anything to do with numbers deployed either since we do already have the limited-use Ta-152, Me-163 and Wirblewind. The Ki-100 would have needed a little extra art and model work than the other two but way less work than a new ride.
Not sure why this is, perhaps the number of rides in the MA is becoming a resource/FPS hog and HTC are wary of adding too many more?