I agree. A sequel would definitely hurt the ending for me, which is the best part of the show, in my opinion.
If there were to be a prequel, seeing more of Spike, Vicious, and Julia together would probably be the thing I'd want to see most. More stuff with Faye could be cool, too.
Even stuff about Jet's past in the ISSP before he became a bounty would be pretty cool, I think.