I use a card system for Luck in my Mekton Zeta game, set in the Robotech Universe. It's an alternate rule that I got from one of the fan community sites, but I'm having trouble tracking it down now. Some of the cards include "Sub-plot" cards, little complications that will grant the play a reward for choosing to include it, that basically give the GM permission to include certain sub-plots that might otherwise annoy or even alienate the player. Like, a case of mistaken identity, or a romantic angle. Or even some sort of "Nemesis" being involved.
I've honest just spent half an hour trying to track it down, but my google-fu has weakened it seems. If I do find it, I'll post a link.
