What punishment could Man City face if found guilty of Premier League charges?

Relegation down one league per each ill-gotten title

What's the 9th tier of English football?

How harsh would the penalty have to be to make their cheating not worth it?

Taking away all their "ill gotten" trophies and relegating them wouldn't do too much. People would still remember them as the dominant Premier league team that won a treble. And in a few years people would barely remember their short visit to x division. It would just be a stain on their Wikipedia page.