You could look at the Family / Clan model.
You create your first two characters as a part of two separate Families or Clans, when you get to marriageable age you create your third character (son / daughter / lizard baby etc) and keep on adding party members to the clan / family. It would follow the empire building model Luckless described, but maintain the RPG character building to a greater degree.
This could work great with any tribal / clan based society (think Vikings) where your 2nd generation characters inherit certain traits from their progenitors.
To keep the game fresh you could also build in something that I would love to see in a game - real wounds, compounding wounds, real death. Every time your character "dies" you select a wound, these start as minor (scars, lost finger, lost toe) with minor buff or debuff and grow in potency as you "die" more often until the only wounds left to choose from are crippling and eventually mortal. For eg. you might get a scar, then a bad scar then eventually frightening scars. Frightening scars would give you a buff in intimidation and possible constitution, but a debuff in charisma and make people either afraid or hateful of you.
Further "random" game events can keep the game play going in perpetuity. You would have to define a large number of these and pre-script them, but to occur at random locations and random times to keep it from feeling like you're just grinding or repeating the same old over and over.
Incidentally, I find that ending a good rpg game is much like finishing a good book. You just wish it would have gone on longer.