Make Concentration Aura useful
it doesn't have to be a big mana cost reduction, by just reducing 10/15% mana cost it is already doing a lot and people will start to take the ability into account, besides you already have "concentrated" green RE's that reduce mana costs, so it fits.
I don't think concentration is currently useless. It's probably pretty unused, a mana reduction would relegate it to healers only probably though.
As a caster I've never taken pushback resistances, unlikely ever to either. I don't PvP so pushback effects tends to be time to move effects. Mana also tends not to be an issue for most non-healers/pure caster builds as they have enough inherent -mana cost effects and mana regens (totems, armor, innervates etc.).
I'm all for making it more useful but currently it has a use and the proposed change is still a very niche use for it. I'd be more willing to support something along the lines of +% SP or something of that nature.