I'm in chapter 13 in my only city. I am the AM of my fs so that gives me something to do besides watch the paint dry. On the other hand, I expect I will reach the end of the lne in the next few months, unless they release a new chapter, in which case it could be as much as a year. In any case, the question is, "what then?"
In thinking a about it here are a few things one could do when that fateful moment arrives. Some have been mentioned by others.
1) Start a new city. (More on this later)
2) Tinker with your city. Efficiencies can be had, improvements are almost always possible. Use the ElvenArchiect to see if you really do have the best layout -- then change it.
3) Join a small and struggling fs and help them become a big and stable fs -- you big city could be a real boost as could your experience and know-how.
4) Tear it all down and start again -- this time focusing on making it a "production" city, a "Fellowship Adventure" city, a "beautiful city" or some other idea.
When you find you have reached the end of the line ask yourself why you played in the first place. Most of us play because we enjoy something -- the fellowship we are in, the sense of accomplishment as we move through the game, or perhaps, the actual creative process we engage in in finding just the right path forward. So why did you play? Whatever the reason you will find it's still there, even if it means starting again.
Now when I say starting over I mean a new city. Now with a new city you can set your self some different goals. Maybe you want to build it faster than before? Reach the end of the research in X months? Maybe you want to build it using 0 diamonds this time? Maybe you want to make it look and feel EXACTLY like the one you just built (which would be a challenge as you'd have to duplicate every building!). Or maybe you want to build a FA city, a beautiful city, an efficient city, or a city with a certain color scheme. The point is, a new city can be just as much of a challenge as the old, and just as fun. So start a new city. That's probably what I'll do when I complete my current one.
AJ