My new challenge is fun because it isn't just your run of the mill legacy challenge, it changes every generation.
My 1st generation had to live by certain rules and the 2nd and 3rd and so on get different rules. And since it isn't a population explosion, I get to really know each sim and actually care for them and help them complete goals. The RKC and such is mostly about having babies within class groups and trying to have everyone so closely related that you almost can't marry anymore, but not quite.
So I am on the 3rd generation of the Random Legacy Challenge. And I'm starting to lose interest again, but only because my bloodline got very black-haired very quickly and I am bored of the lack of diversity. My founding sims had a male (Adam) with red hair and a female (Eva) with black hair, so of course their only child (Skye) had black hair. All 4 of Skye's kids (Ella, Kylie, AJ, Alexa) had black hair. And my heir, Ella, so far has 3 sons - all with black hair. Ella has 2 generations of red-headed men in her (both her father Daniel Pleasant and her grandfather Adam) and I made her marry a red head (John Johnson), but those dark-haired genes are STRONG! I have now kicked John to the curb and married Ella to another red head (Komei Tellerman) and only time will tell if their children will futher infuriate me.