One of the things that seems odd to me is that there isn’t much in the way of midline support characters. I think Ronin and Fortress are the only ones with support skills, and Fortress isn’t exactly game breaking.
It seems like backline is thematically mostly support, but they have some good DPS options, while frontline is mostly tanks, but they have some good support and DPS options. Midline, on the other hand, is almost entirely DPS characters, with some disruptors that I guess you could slot into the support category if you squinted a bit. It seems like something that could open up a few more builds if built out.