I am currently undergoing the Through History Legacy Challenge
I was attempting the original Sims Legacy Challenge by Pinstar - I got to Generation 2-3 and started getting distracted. It still exists and I may return to it, lol.
Unattempted challenges of interest :
Disney Legacy Challenge - REALLY EXCITED. I'll do this as an epic story AFTER Through History.
The 100 Baby Challenge
101 Dalmatians Challenge (page link?) - 100/101 babies... but with puppies.
Adam & Eve - very expansive legacy, become greater than the Goths.
Triplets Legacy Challenge - Own idea, but difficult due to slim chances of triplets, and they'd have cute names like 'Prudence, Constance & Temperance'.
Accident-Prone Experiment (page link?) - An idea someone posted on The Sims 3 Facebook Page is to create a Sim with traits such as Unlucky, Clumsy, Absent Minded, possibly with a trait like Dare Devil, and see how long it takes for them to kill themselves on High Free Will mode.
Any one know any others?
I heard of a Civil War Challenge a bit similar to the one I'm doing now - make a Civil War Era town, all male Sims go to war, you add things as they were available in real life.