The Wespe I'd vote for and the Pershing. With the Wespe, HTC must feel that land based artillery would dramatically change game play and therefore unintended consequences of such changes must be considered. Given the amount of times I've seen that stated it must mean that HTC has always taken great care not to implement something before it has been completely thought out to most conceivable ends.
The Pershing
However, as per the amount of threads about it, the J2M has been on the wishlist for 10yrs and imo would be a great addition. It would foreseeably get heavy usage with its 3 variants.
J2M before anything else, please.