Why doesn't the Premier League (PL) just ban them from the competition from x number of years? For example the number of years they are found to have been guilty of cheating, so like 9 years?
If the league does that, it doesn't matter then to the Premier League what happens to City - they'd need to figure it out for themselves with the Football League (FL), but that wouldn't be the PL's issue as they have no ruling over the FL.
City would then need to apply to the FL and see if they would be willing to literally add an additional space for them in the Championship, which hopefully they wouldn't, but who knows given the league would stand to make money off of City being a part of it.
The hope would be they are not given special treatment to join the Championship, League One or Two, and they'd need to find themselves lower down the pyramid.
Obviously they'd win consecutive titles (and still play in the Champions League in the first season on account of having just won the Premier League before being thrown out most likely) to get all the way back up, but it would still mean they'd likely end up winning the Championship about 4 times with the promotion spot being passed to second place and each space moving one down as they'd still be banned from joining the Premier League.
To me, that seems like a very fair punishment for these crimes. Alongside of course some substantial transfer ban, removal of any honours won during that time plus other punishments on top.