You might be underestimating how difficult it is to make a mod - especially for a game where we have no idea if the source code is going to be released.
Even after the technical programming aspects are taken into consideration, balance is a massive issue, and I'd presume AJL would want to reintegrate a class-based system into AvP3 if he ever did the mod. AvP2's existing class system was a major help, as it provided a (very) rough basis for balance.
