I'd join a thief character for sure!
Anyway! Mona, I can offer guidance if you like. x3 Basically, waaaay back in the day when SotE was new, I ran the Thief's Guild with my character Sachi. You just really have to be proactive and organize missions, and you have to make sure everyone is active and poke them a lot. It also helps if everyone involved has like MSN or something so everyone can discuss plot ideas and stuff! But yeah! Missions! Those are fun! Back then, there were TONS of plots with the thieves and they were a really active group.
Basically, the way I set it up was Sachi would get the thieves together and they'd coordinate missions. They did indeed sneak into Serendipity's castle back then! That was quite fun. x3 They also poked their noses in a lot of plots and events. For instance, if there was a ball in a kingdom, they'd be there to pick pockets and cause trouble.
They were a very "steal from the rich and...give to ourselves >_>" sort of group. They were also very tight knit, so there would be a lot of group plots and missions, and then there would be interpersonal ones as well. And the thieves eventually, at one point, were even heavily affiliated with Serendipity's nobility. xD At one point, the queen, Dawn, was the niece OF a thief.
So I think it'd be fun, and I'd LOVE to see the group active again. I just feel someone else should take the reigns! I played the leader of that group for about...jeez...four years. So it's time for new leadership! x3 But again, if you need help, I can offer it!
Also...again...I can't stress it enough. MSN. Everyone should exchange that. It helps. Heck, I think that should be a general thing...x3