The whole "but there's nothing meaningful to add" mindset is ridiculous, the game is barebones as f*ck Moderator Action: Please watch your language. , even with religion added in G&K. You can take a look to the suggestion forum, but there are a ton more mechanics, not units nor more content, but rather entire new systems that could be added in order to balance and add flavour to the game:
- Supply limits in order to limit armies and perhaps introducing limited stacks
- Corporations as a means to twarth enemy victories
- International trade routes and advanced commerce systems rather than the current fisher price type of economy
- Apostolic palace / UN, something that would make diplomacy more interconnected and increase the "butterfly effects", something that the game is sorely lacking right now
- Colonies and vassals as a new means to defeat a civilization without restorting to the horribly broken puppet state mechanic
And you can add far more new civilizations as well: Portugal, Mali, Indonesia, Kjmer...