Hi all, I saw a reference in the Answers thread to settlers who can be improved for the late game.
This sounds like a great idea to explore, here are the first thoughts off the top of my head:

1) Have tiers of Settler unit, where Building upgrades etc. would provide access to the more advanced units. The more advanced settlers could create a town which automatically has a Smithy and/or Granary etc.
2 ) When building the Settler unit, you could choose how many units of population to invest into it from the originating city. This would allow the newly-founded town to build up more quickly.
3) In the original city where you build the Settler, select a low-level building and say "Deploy to Settler". The resulting settler unit's cost is increased by the cost of that building, but it will then create a town which has that building automatically (...I'm not sure what the story-logic behind this one is...)