For me it's a bit demotivating to know that there will be always a next level. With that in mind you can come to the question about a target in the level system. The solution to this could be a special supreme level at level 100. To make this level special it can have a special name (for example level god or level king instead of level 100) and the player get a crown symbol besides his avatar. The resulting problem of this supreme level would be that the few players who have reached it will have less motivation to play the game. This can be solved in that way, that additionally to the crown symbol the posion in the all-time list (career list) is shown besides the avatar. Conditioned by the fact that playes who reached the supreme level are the same playes who are on top of the all-time list, to show there position in the all-time list besides the avatar would gave them a motivation to advance this all-time posion. So the playes who reached the supreme level would still have a motivation to play this game.