As a radical option, might make sense to have a player play a civilisation rather than a nation. Kind of like what was done in AFSNES. Changes up to the level of ethnic composition and religion (to say nothing of actual nations) are in, but some cultural continuity is preserved as a norm. The main problem is that not even I have any concrete idea as to how it should be done exactly.
Somewhat more feasibly, how about using the stability stat (well, maybe a better name would be in order, but I'm not sure as to which) to impose your will give the players a fair warning? I.e. at first a nation is ascendant, then it might start to stagnate or enter a golden age (depends on lots of circumstances, including player actions ofcourse), and eventually, in all the probability, come into a period of drastic decline. At which point a player might try to pull through a renaissance (won't always work, ofcourse; the Egyptian 26th dynasty obviously tried and as obviously failed, for instance), or switch to another nation, or oversee the collapse and move on with one of the successor states (frequently compatible with renaissance, but again, not always).