I think this would add a huge amount of strategic depth to gameplay (PvP and otherwise).
Allow us to specify whether a heroes is placed in front, mid, or rear-line. Of course if there isn’t enough space on one line, then just push them forward/back the way it works now.
This would allow players to compose a larger variety of teams just by shifting the positioning of heroes and greatly expand the strategic possibilities. It’s fair because all the positions are equally available to everyone.
It also gives PvP more depth because then you can’t easily predict where a specific opposing hero will appear. Right now, as soon as I see the lineup on the match-up screen, I can more or less guess exactly where a key hero will appear. Of course that’s good for me, but it would be more fun if I was kept on my toes.