It requires exponential levels of labor for every cosmetic

Unless all games use the same models and shit so the cosmetics actually are directly transferable because we're only getting "generic variant of the One Game every game is now a generic variant of for the sake of content portability". If you get too creative with your designs, you can't participate in the universal cosmetic sharing and mutual marketing thing all the cool kids do.

I wonder whether the engine will actually lock down your range of options to prevent that problem? Or maybe just heavily discourage developers from not implementing the "One Game" stuff. I'm both put off and curious to see how it'll develop.

legal agreements with disparate game owners

Actually, the "stamp all your stuff from the same mould, and because we own all the things you stamp from that mould, we can freely share them between games and also hold your work hostage" approach would solve that too. If it all becomes too generic to copyright individually, whoever holds the rights to the One Game has freedom to do with his games whatever he wants.

Sure, I make an asset in my game for a skin the player bought in your game, who keeps the money?

The longer I think about this, the more I worry that my One Game joke is the only way to actually make this work, and also very desirable for Epic. That would make it less of a joke and more of a foreshadowing.

Because I'm pretty sure the answer to that question is gonna be "the owner of the One Game". Whoever first published that skin will get a cut of the sales, but since they're so easily transferable across the standard objects you're expected to have used, you don't need to do much to have it transfer anyway. Besides, your skins also get shared with other games, making for free advertisement!

Unfortunately, one game made Nazi uniforms, Epic refuses to ban them because they make good money off it and now you've either got Nazis running around your game or you need to somehow get out of the asset sharing thing.

Okay, now I'm horrified at the prospect of how this might develop.