With the understanding that the Spitfire XII has been a passion of mine for 30+ years and I'd love to have it.
It's not a top ten however with the Spits we have already. I wouldn't turn it down if AH adds it, but only 100 built and it served from Feb 43-September 44.
The biggest missing piece is the Beaufighter. There is no other bird missing from AH that was as big of a player in WW2 as the Beau. 1940-45 and active in every theater of the war outside of the Eastern Front with the Russians. It's particular abilities fit completely with AH in that it was a fighter, fighter bomber, rocket carrier and torpedo plane.
In terms of armor, I'd love the Panzer III Considering it being the main German tank during the Blitzkrieg, in Barbarossa and in North Africa, it's a glaring omission. That being said, it would probably be a hanger queen in the MA as folks want big guns. For events it would be a wonderful addition. If it arrived then a comparable British tank would be important. Most likely the Matilda would fit as again, it was there in France and in North Africa. It went to the Russians in Lend Lease as well.