I would add an extra turn card or two, just because you have no haste enablers in blue, so an extra turn spell would let you make a ton of tokens and then win without having to wait a turn and hope you don't lose your board

That's a valid point. I like that idea, but Urza is well known for being pretty degenerate and extra turn spells are the kind of degenerate nonsense I'd like to try and avoid in the interest of making Urza somewhat "fair". I do have Karn's Temporal Sundering somewhere, though. It's flavorful, and it gets exiled after I use it, at least, and requires a legendary creature to cast. I may try and find space to add that one in. I'll add a few more of them to the maybeboard in case anyone else would find them useful.

Thanks for the advice!