Way back in the day, I proposed that any 2 cities that reached each others borders, as they can and often do now, have the ability to be Merged into one larger city base.
It would be cool to take 2 cities, that when the touch, would offer you the chance to take, for example, a L2 and a L3 and Merge them into a Single L5, which then gets encased within whatever the lowest current wall tech was researched while reaping the combined resources that had been accumulated by both.
Of course there would be some Building based Logistical issues, but given the news of the upcoming 1.1 multi-building allowed rule, this would only be an issue for those 1 off's left over under the current prevailing L5 city rules. That could easily be resolved by allowing the player to choose which city structures would be retained/razed that are duplicates, if any even exist.
Anyways, something to contemplate going forward I guess.