Make him DC's Scourge. EZ-PZ.
Jason's the point man for a mysterious wealthy backer, (either a retired superhero or supervillain) who goes after super criminals, capturing them if possible, killing them otherwise.Make Joker his first kill, (and let him stay dead for a couple of years). Supporting cast could be his support team, (like an Oracle and an Alfred), plotlines could be Jason trying to figure out who's he really working for, and figuring out what he really wants with his new life.