Ok, maybe to match "IRL" they should come in together --
BUT
Both the Wirbelwind AND the T-34/85 are essesntially mods of current vehicles, requiring far less work than a fresh Panther. These new vehicles probably require less development time than ANYTHING that's likely to come out of the shop, so they represent great additional gameplay at relatively less programming "cost"
IMHO, these 2 -- Wirbelwind and T-34/85 -- are excellent choices for the next additions to AH2.