Okay, just change to 15 cities and call it a day. And have the AI build units at 110/95/90 or something instead of 120/110/100, to make the AI somewhat stronger militarily. Make them pay something for upgrading their units as a compensation for the slight strengthening of early civs.
15 cities is enough for a nice empire with a semi-historical city density, like
1. London
2. Plymouth
3. Manchester
4. Edinburgh
5. Dublin
6. Toronto
7. Vancouver
8. Cape Town
9. Durban
10. Madras
11. Calcutta
12. Bombay
13. Sydney
14. Darwin
15. Perth
I don't think that the game should punish the player for that. Of course, if the player wants to go on a wide-tile conquest spree,
1. Southampton
2. Aberdeen
3. Dublin
4. Paris
5. Marseilles
6. Bordeaux
7. Valencia
8. Lisbon
9. Rome
10. Athens
11. Constantinople
12. New Orleans
13. Sioux Falls
14. Durban
15. Madras
He's welcome to, it's not like controlling all that with good stability is prevented by the 10th city rule.