Upgrades. And something unclarified about its ability to ferry infantry.
Considering I've seen official screenshots with a square-nozzled "MG" for the Universal Carrier, and the official art for the same vehicle showing a squad of Tommies in the Carrier with one of them toting the Vickers K MG, I infer that the Universal Carrier will get an upgrade to become a Wasp, and crewing the Universal Carrier will give it an extra MG in the form of the Vickers K.
It was rather obvious that the Universal Carrier would get a flamethrower upgrade since the Royal Engineers don't get one (and that the Universal Carrier is the only vehicle of the non-doc type that could get one), and 3 out of the current 4 factions get flamethrowers.