could you do second upgrades?
if yes I may try to port the city into ERA, using script similair to dragon peaks. Yet still I didn't tried something like this. sadly in this approach there would be no additional heroes for oasis.
Background: I am doing huge mod which mainly adds creatures, first additional upgrades then neutrals. I didn't incorporated towns yet but I know that after training I would be able to. there are two types of new town i may include - Neutral town with all dwellings built, normal towns which have buildings to build and have additional building(s) to upgrade all 7 levels of creature one upgrade further (base->upgrade->2nd upgrade).
PS: if you wish to release it under VCMI tey still support 2nd upgrades.
PLZ thought supporting it