I've started one! I'm not playing by the legacy rules, though. I've made up a storyline for them and don't want to be tied down by the rules of a real legacy challenge. Im also playing on LONG lifespan PLUS some added customizations to the lifespans to make them even slightly longer. (Thanks MCCC!) As a result, it's slow going. But I'm trying for 10 generations. Not sure if people would consider me on gen 1 or 2...but...
I started with a couple in the adult life stage with a teenage son. The teenage son is now married with two toddler daughters. The older daughter will be the next heir. I consider the teenage son to be generation one...not his parents. I just wanted to start the storyline with him as a teenager still. So, even though I have grown to love his parents, I consider his daughters to be generation two only. It's been so fun so far. I love playing with giant households. Right now, with mods, I have 10 Sims in the household.
