I don't think something like this would work in a sim league but longevity and potential are a couple of interesting concepts.

Let's say you start with 0 out of 20 in both attributes, and you can put up to 30 points into longevity and potential before your 7th season. So for example, you could go with 20 potential and 10 longevity, or 20 longevity and 10 potential or 15 in both.

If your potential is above a certain threshold, bonus points will be added to all of your attributes during your prime years (seasons 7-10), and the higher your potential the better you will be at your peak.
Longevity determines how many bonus points will be added to your attributes in seasons 10 and later.

So you can build a player that will destroy the league for a couple of seasons but will decline quickly, or you can choose to build a player that will never be a true superstar but will be very good for a long time.