Okay, I've worked out something for the Timurids, ironically using code I wrote to get the original ability working. It'll probably be a temporary UA until the next version and I work something else out.
For those curious, the ability I was trying to make was essentially:
"Captured Cities have a chance to retain Unique Buildings present in the City and captured Unique Buildings are constructed in your Capital."
And actually, we got that working (thanks in no small part to the help of Framed Architect and Moriboe). It was a really cool unique ability which was also very historically appropriate.
However, then the game wanted to be completely random about how it treated building classes.
First, say you captured a Mughal Fort, this didn't immediately prevent you from building the default Castle in the city. However, this was easily fixed in lua.
Then wonders giving free buildings basically overwrote stuff, and this couldn't be fixed as there's no setfreebuilding in lua.
The real problem though, came with building pre-reqs. Despite it being called <Building_ClassesNeededInCity>, it doesn't actually check the building class in the city. So say I wanted to build a University, you need to have a Library in the city; it wouldn't work with a Paper Maker (unless I was China). The really odd thing, though, is that GetBuildingClassCount does actually check the building class. So, basically, the code is inconsistent throughout. Anyway, I actually fixed this problem with lua as well. However, when it came to the same issue with National Wonders is where I hit a brick wall and it just couldn't be fixed (we'll I did devise making a dummy version of each National Wonder just for the Timurids, but at this point, yeah).
So there will be a Timurid update shortly with a different ability. Maybe if I ever do a big modpack I'll do a dll and re-implement the original trait.