I've thought about putting together a bid for a gTLD, but operating it like every other username-based system. Once you register, you own the domain for free forever.
It's expensive: > $200,000 to apply, ~$50,000 / yr to maintain, plus $0.25 / yr per domain. [1]
You could run such a system on ads, but you could also do the Let's Encrypt / completely open route where you raise money and build up an endowment to run in perpetuity.
Imagine how cool Twitter would be if you had "username.twitter". Or could email "whatever@username.twitter" (or "username@twitter"). Same with gmail, etc.
I've also thought about registering one in my first name and keeping it to myself. It's super nerdy and way cooler than an NFT or ".eth" domain.