Cosmetic changes like: different faces, different dynastic coats of arms, different hats for the portraits (sound familiar anyone??), possibly different icons for the towns/castles/churches of various cultures and religions, different sprites for units.
All things that *could* have been in the main game, but that don't fundamentally affect the game play. The game would be playable (if a little bland) if all men had one face, all women had another, and all children had the shadow placeholder; and dynastic shields could have been completely ignored - they aren't needed at all. Before anyone says it, I am glad we have the various portraits and shields, I merely use them as examples of things that are not necessary to the game... This is what I meant by cosmetic DLC.
Functionally, anything that merely changes how the game looks, rather than how it functions can reasonably be in the category of cheap DLC. Anything that impacts on the engine or ruleset should be a patch to repair non-functional mechanics/exploits, or an expansion if it introduces new things. The patch should be free, the expansion should not.
I know what you mean by cosmetic changes, what I'm saying is that there are some basic cosmetic characteristics that must be in the full game in order to make it appealing. Adding something as important as a Mongol race and historical dynastic shields (If you give us historical start dates, having historical shields should be a basic) as a cosmetic DLC doesn't seem good at all. If it was a cultural separation between Scandinavians and slavs/european, then a cosmetic DLC would be cool, but if it is about adding the Mongolian characteristics (which I believe hardly share any common looks with the eastern cultures present in the game) then I believe it should be in the full game.