It's being rumoured that John Constantine could be part of team in season 2, now baring in mind that CW haven't agreed to a second season yet so should be taken with a pinch of salt till its confimed. I did say a few posts ago that I felt he would be a perfect fit for the team and the producers have said that if the show is given more seasons that each season they could change up the team so here's hoping it does happen.
that would be great if does happen, I think cw should do 10-13 ep mini series for some of the characters every so often so that the viewers, new or old to dcverse, gets to know the characters plus could set up some interesting plots for shows. a Constantine v angels mini series (slight continuation from the original series) would be great plus could tie into any show.