I'm just popping back on to gaia for the first time in (half a year? Maybe more, maybe less?) and personally I was overjoyed to find I hadn't been kicked out of this place. I didn't announce my leaving because I didn't really feel there was a reason or place to do so, but one of the most frustrating things for me and other working students I know is having to re-join guilds (or if denied, find new ones with the same community and content we love) after fortnight-long to yearlong periods of inactivity. Just letting your members stick around is a great first step.
As far as incentives for people who are already on gaia, think about stuff we already have. We have a birthday thread, right? So how about on each member's birthday, a staff member creates a Happy Wriggling Day _______ thread with the member's name, a few facts about them, and a link to their wishlist? How about a guild announcement "newsletter" every month or something with an update on the standings of current RPs, contests and giveaways?