Your point on variety is very well-taken. One thing that helped was to list all of the proper nouns and categorize them as presently constructed. I found that of the 33 proper nouns, five of them were sports names, which even for a 21x21, seemed to be a bit much since that was not the theme of the puzzle. So after doing a lot of editing, I've cut down the number of proper nouns to 29, with now just three sports names, which seems reasonable (and I was able to get rid of one pretty obscure term and replace it with a common English word). I still have a fair number of geographical terms, but they're at least part of a diverse selection of cities, countries, bodies of water, etc.
I agree that certain entries can act as either a proper noun or as another term, but after reviewing what I have, there's really no way to clue them except as proper nouns. Now that I have 29 instead of 33, maybe that will make the grid less of a trivia contest.