So here's one reason to think of all the cards as nouns, not proper nouns: THEME.
You are a monarch, like your parents before you, a ruler, of a small pleasant kingdom of rivers and evergreens. Unlike your parents... You want a Dominion!... To do this you will hire minions, construct buildings, spruce up your castle, and fill your treasury.
"I have multiple City cards in my deck" is not thematic. "I have multiple Cities in my Dominion!" is more thematic and more fun. "I have multiple Citys!" is just silly (or is it Cityes?). Getting into the theme, the capitalization is for emphasis and importance (in the spirit of historical excessive capitalization of common nouns) of the things you own in your (imaginary) dominion, not to denote a name of a playing card on your kitchen table.
[In the MTG case, Cities of Brass does not mean multiple City of Brass cards or multiple cities that are named City of Brass, but you own lands which have cities made of brass on them. And they're important. With capitalization for emphasis and normal pluralization, it's all about the THEME!]
In my Dominion I build Cities and Wharves and Embassies, hire Goons and Nobles and Smithies and Thieves, and clear out infestations of Rats and rebuild Ruins. And every person, place, and animal is important to becoming the dominant monarch.
The fact that Donald views Ratses and Ruinses as the correct plurals just means he's not getting into what little theme there is when he plays (or he's secretly role playing as Gollum and Dominion is really set in Middle Earth).