Out of curiosity, is that temple using one of the game's existing art assets, or did you add it? Looks great, btw
As far as brainstorming - you've set a bit of an exclusionary filter by specifying that this is trying to re-create a game that many (most?) haven't played...entertaining to watch the progress though.
If I may be so bold as to make a request/suggestion - perhaps you could divvy up the things you're creating ala-Children of Storm so that once you're done, people could pick and choose pieces? For instance, I might want the extra items, but not especially care to add all the new factions. You might even get people to playtest parts of it sooner if it can be piecemealed out vs. a total game conversion type of thing.
Obviously that requires some extra forethought and effort on your part, though, so feel free to cheerily ignore the suggestion.
BTW - on the subject of faction unique buildings - I believe Children of Storm's Lizardmen faction has the ability to build a unique dragon-cave (I think it uses the sovereign resource idea you mentioned above.) There are other faction specific buildings too - e.g. the Living Stone build special Fire Temples. If you haven't already siphoned off all the ideas from there