This is somewhat difficult to decide right now. Depending on map parameters, difficulty level, preferred victory condition and such, a restriction to 5 cities may or may not have a large impact on the game. I do fear that it will very much limit the options we will have in playing the game, in particular that (in most cases) it will necessitate fairly "optimal" gameplay. Also, people who have played this (I haven't) say it's boring.
I do think limiting number of cities might be interesting, mainly because it will leave some land for the AI, but maybe 5 is a too small number. Maybe it should be tied in with corruption levels or so, I don't know.
For this reason I will vote "no", somewhat reluctantly. It's really too early to tell, I think. Maybe we can wait a few days for our citizens to come up with other interesting variant rules....