One of the wonders in the expansion is the Great Wall, and one of its effects is "When you lose a battle, your opponent gains 1 less loot." Incidentally, the base game rules categorize the Military Victory as loot (page 25).

I know it's a bit early to be asking since the expansion rules aren't even released yet, but if I conquer an opponent's capital while they have the Great Wall in another city, I don't win the game? Sure they could just obsolete it or capture that city first, but it still seems like an odd edge case.

I very much doubt that you should read that in such a way as to regard the military victory condition as 'loot'.

Page 7 of the expansion rules clears this one up nicely....

Although the losses incurred by the losing side remain
basically the same, the looting system has been adjusted
to simplify it and to avoid certain abuses of the game.

If the loser had one or more figures in the square:
The winner gains 1 loot.

If the loser was defending one of their non-capital
cities: The winner gains 2 loot.

If the loser was defending their capital city: The
winner immediately wins the game with a military
victory.