Posted 30 September 2012 - 06:12 AM
For skills, look at GTA:San Andreas or sports games. Skills have a forced balance to them, you cannot be a master of everything. There was and MMO that i read about sometime earlier this year that adopted a "Use it or Lose it" approach to skill leveling. This will prevent any player from becoming maxed out. Eliminate class and let players unknowling determine for themselves where their strenghts will be based on how they play. The more the use a skill, the stronger it gets and the opposite.
With that in place, players will have to change their play styles to combat differing challenges. You may also think of having the game scale or morph it's challenged based on the player, enemies will become stronger defending against the player common tactics and will attack in ways that exploit the player's weaknesses.
If you wish to keep a player playing after the final quest, using multiplayer is usually your best option but you could also have challenged produced by the game in reaction to the player. A few indie games i've seen continue forever becoming harder based on the player. "Bosses Forever."