it seems the most significant limitation is in regards to modding the AI
That's the DLL. And maybe there are so few 'large modders' precisely because they can't make a 'large mod' because there is no DLL access.
I jumped into this conversation because someone said Civilization VI is not mod friendly, upon which someone else commented that it actually is.
Well, no, it is not. Maybe on its own, but compared to the usual standard of Civilization. We've already addressed IV, and III includes a whole GUI that combines map editing with modding everything, and it has modding directly done on its .exe - even if you say 'okay but it does not have the equivalent to DLL access', then I'll still say that by your own argument a GUI is more mod friendly than a bunch of textual files. I don't know about V - and I suspect you are only comparing VI to V - and so VI may be mod friendly compared to V, but it is objectively not compared to IV, and subjectively not compared to III.
It seems that a lot of examples are 'you couldn't mod X in V, now you can in VI' - yeah, and you can mod literally every X imaginable with DLL access, including making the AI understand it (because even if you can add whole new features to VI, effects that don't exist at all within the XML files, then the AI wouldn't know what to do with them without being able to implement these features not only through Python or whatever VI's equivalent is, but in the DLL).
So, mod friendliness is indeed a valid thing that can be improved, and will do a lot for the longevity of the game in general (after all, even III is still reasonably popular) and also to fix the lazier and lazier attitude that Firaxis has attained after IV (even here I see mentions of the AI flat-out not working with e.g. Monopolies - let alone that the AI in general is very passive, incapable of handling 1UPT, and all in all just much worse than it was in IV or III).