another idea for unplanned settlements. these dont involve settlers. the byzantines built a long road through no mans land to connect their capital to distant cities. i find it useful so i build a couple of forts and station units along to detect and defeat barbs. in real life eventually towns would build up to service the forts and the caravan and other traffic along the road...not sure exactly how it would work but i am thinking if the forts are built and then a unit fortified there for a number of years it should become a city. it should happen slowly for ordinary forts and maybe quicker for citadels, but they cant be in any civs territory at any time. so if someone settles a city there and it expands its area before the fort becomes a city the process is halted. or you can leave the fort unattended for a turn now and then to reset the counter if you dont want to found a city there at all.