If you want a counter to be truely effective, it needs to happen in the Rankup phase before the open/startphase.
Units with a Rankup condition could work, but maybe there's a way to play Grimoires with a Rankup condition aswell?.. Meet certain requirements on the field/Spherelvls/whatever to have an effect in the Rankup phase.
Wether Returning is balanced or not isn't even the biggest issue. When half of the Top10 suggestions in a suggestion forum are about changing/banning/countering returning, that's an undeniable sign it's not enjoyable gameplay for a large(st) group of your playerbase.
Control can be so much more enjoyable and better to the gameplay then this returning or engaging fest.
Why on earth keep more people frustrated then satisfied. Why on earth do you nerf everything around 1 mechanic even the people PLAYING it hate? I can only imagine the designteam has no idea's on how to move forward or backwards. Which is a real pitty...
Want basic ideas? Units with "Friendlies on Spot -X- get MHP/Agi/Atk/Rng/etc". "V formation (spot 2-4-6) gives bonus X. Y formation (2-5-7-9) gives bonus Z". "Unit(s) on Spot -X- get Rng=0/Engaged/Agi=0/HP-20/etc". "Closed unit on Spot 9 can't be ressed". "Do [dmg: Atk target unit] to Unit in collumn behind." "Chose a unit in 257. You get SP = Spot - Base Lvl of chosen unit". And that's fieldpositioning stuff. Don't get me started on stat switching ala Snaf/Noze! A whole world of control opens when you can reorganizing stats on a field. Clone HP, switch Def values with a stat, switch Atk between enemies, turn Rng around, go crazy?
Balance is up to testing, but coming up with mechanics that are a ton more fun then simply 'click to send away' should be damn easy. Stop hitting around the ff'in bush with OwlSage Rng or Mirelia MHP nerfs and actually start making changes?