In Civ5 there is also the option to buy land with gold. I think that's interesting but on the other hand too exploitable... with gold in C2C so i suggest you can produce culture to buy land with it later.
With open borders enemies could buy your land (much like orthodox settlers do with palestinian soil), so you might have a need to close borders sometimes (which I never do in games right now)...
The build culture process is not used for the most part of the game but with this it could get some more value, also the meager and lesser culture build.
How could a tile be bought?
Imagine the cursor getting like when you make a sign on a tile (ALT+S). Once you have enough built culture accumulated the field you focus on gets green (or any other colour) which means upon clicking it becomes part of your culture. The choice will be able to be done at beginning of turn or as a mini-symbol on the right. To have the tile picked near the city the culture was built in, only around that city tiles highlight as buyable. Maybe done in city UI as well, lot of possibilities with that.
In worldbuilder there is some code that allows you to pick if tiles are already either seen or unknown to the player, maybe it can be used.