I like the idea of converting buildings at higher level stronghold, but I would like to propose a more robust set of Building conversions and features, which might address some other issues frequently discussed (e.g.: ascension items).
Farms become harbors (discussed in a prior post), which are better food production.
Houses become barracks, which operate just as barracks do now. The only change is that ONLY houses become barracks.
Training grounds become arenas. Arenas can be used to create higher star heroes from lower star heroes. For example 10 x 1* heroes might go into the arena and create a single 2*. Max * available is a function of arena level.
Mines become power plants and forges become alchemy labs. Must be done jointly because power can’t be stored (unlike iron and food) and labs don’t function without power. Alchemy labs create ascension items, either from raw materials or from other ascension items. Again, leveling up plant/lab combo (comparable cost to stronghold leveling), allows higher level ascension items to be created.
Storage facilities are not converted, but everything else allows specific, and related Building conversions.