They could always bring back Willoughby Kipling, whom Morrison created for Doom Patrol because he couldn't use Constantine. It wouldn't be the same, though.

I doubt Constantine will be allowed back into the DCU anytime soon. The longer Hellblazer has gone on, the more John's character has become associated with rather heady situations, like Royal Family members being possessed by demons, corrupt Church officials, bisexual kink, and other things that DC wouldn't feel comfortable about letting underage readers (and their parents) get hold of.