A story line that actually progresses is HUGE. Especially when it progresses in ways I don't expect.
I try to leave a "hook" for imms to use when I write my roles. Basically I put something into the role that is unknown to the character so that some imm can fill in the gaps with whatever he can come up with.
For instance, with Ageryn it was all about finding his sister, from whom he'd been separated as a teen. Was she alive? Was she dead? If alive, where? Various evil imms tried to tempt me to do stuff by offering to tell me. Whenever there was a contest and an imm offered some reward, like those orbs of knowledge that give you skill improvements, I would try to trade them back in exchange for info on the Sister.
Of course, for this to work a staff member has to read your role and become interested in advancing the character's story arc. Which is a big ask.
*Usually* my characters become more or less stagnant. I struggle to come up with role entries, since nothing of import ever happens. I kill some people, sometimes I die, etc. That's basically it.