Then there is also the distinct appearance challenge you could do.
IE:
When creating the founder in CAS choose a skin color that does not happen naturally and does not exist already in your town on some other sim. (For example Blue) For any Sim within the bloodline to become the heir, they must also have that same skin color. Any Sim within the bloodline that does not have that skin color may still earn points as per the standard rules, but just may not become the heir.
Will your Sim's family remain a minority in the town, or by generation ten will this unique trait have become the standard? I don't know if the game uses recessive genes or not, but I can see it now if they do... your family has four children before having the fifth one have the chosen skin color. The four spares stay as long as they are useful in the house, but as soon as they no longer have a use you move them out, and they have their own lives. Next thing you know they end up having children that have the skin color because it managed to skip a generation. HAH then it would get even stranger as you get near the 10th generation and your family has spread to fill up most of the town. Your little 9th generation Sim wants to find a Mate to finish the challenge, but you can't because everyone has blue skin now, and while the game might consider them distantly enough related to not be related... you at heart would know them to be distant relatives. Could you bring yourself to have a blue skin mate with a blue skin even if the last time they had a common ancestor was back in generation 1 when great grand-dad had 5 children?