I love this idea. Maybe there could be three pools of tasks- easy, medium and hard, and they could be used like so:
Tier 1: Only easy tasks
Tier 2: Easy and medium
Tier 3: medium and hard
Tier 4: hard
Though there might need to be a way to prevent duplicate goals (no "reach level 5 painting" goal twice)
If one wanted to get even more fancy with it, maybe the various goals could be weighted depending on traits. A geek with the writing aspiration might be more likely to want to publish a certain number of scifi books, a perfectionist might want to make a certain number of high quality items, an evil sim with the master vampire might want to use a malicious power on X number of sims, etc.
That last one could be a lot of fun, giving the player more reason to use certain powers.