What’re the guidelines for what spells should be on its list?

I have MANY questions, but just in case I can’t make it:

What’re the guidelines for what spells should be on its list? I’m hoping to add some XGtE spells to it. That one's easy: I plan to include a list of those spells in a future UA. 😘

— Jeremy Crawford (@JeremyECrawford) March 5, 2019