Considering how precious perks are in Requiem, here's my controversial opinion: why not just incorporate agile spellcasting into light armor by default? Get rid of the "useless" perk.

This won't be OP. Why? Because clothes will always rein supreme for mages. Because of the wide array of mage robes in Skyrim.

Magicka regen will always be better for low level mages than a little extra armor. So even if light armor had no casting penalties, I would still wear robes and just avoid getting hit. But it's nice to give light armor spellswords the option of wearing light armor without feeling like their gimping themselves (except out of magicka regen of course).

And by the time you can access vampire armor and Harkon's armor, you'll probably already have high level alteration with perks as a mage, making the vampire armor a weakness, not a strength.