A lot of people post on this forum about wanting more unique gameplay for Elders. One way to do this would be for your Sim to acquire a new Trait upon becoming an Elder. People don't stop growing and changing as soon as they hit 20. It would be a great way to distinguish Elder characters and gameplay and give players something to look forward to when their Sims are older.
While it would be great to add it as a full extra Trait slot in CAS, I believe the developers have said that the UI of CAS is difficult to edit. It can be done - we saw it with the gender options - but so far they seem resistant to the idea of adding a new trait slot.
Instead, new reward traits would be unlocked when your Sim becomes an Elder. This would mean that devs didn't have to change the UI in CAS. The more positive traits could be more expensive than the more negative ones, meaning that the way your Sim lived their life would have an impact on what sort of new trait (or traits) they developed as an Elder.
Here are some sample Traits:
Feel free to add your own ideas of Elder Traits to the thread.