I'm not sure if you know that the binoculars on offhand can be used to manually aim spells but your 2H method would have been a better way of implementing it.
Yeah, I've known about binocular aiming for a long while. The ability to aim and (crappily) block might be enough of a reason to two-hand, alongside the absence of shield encumbrance, all without affecting PvP in any way (the notion of zoom-aiming in Dark Souls PvP is laughable, even using a mouse). A minor casting speed increase would probably also be appropriate.
The notion that bonuses for magic setups other than one-handing would be OP is dumb anyway. Magic is already extremely underpowered as-is. I don't care if you can deal 1,000 Great Heavy Soul Arrow damage with a +5 Faintstone-infused Staff of Wisdom and 60 INT (unless the target is resistant, that is), because those are 50+ points that could be used to wear tank armor, carry a blast door shield, have more HP and Stamina, and smash things with a bus glued to the end of a telephone pole... which ends up being much more workable and safe in most situations, unless you're using NPCs to tank bosses while spell-spamming, which is the same as admitting you have to basically press a skip button to get past the boss.
In fairness though, high INT also allows for equal use of pyromancy, which is retardedly strong against invaders and hordes of minions. It lacks staying power to actually make one's way through a level while relying on it, but it's good.