Would it be broken to make a feat that lets me use the sheath of my sword as a shield...?
You shouldn't need a feat for it. It's a shame that they lost "buckler" along the way, because I think ANYTHING reasonable should be able to be used as a +1 shield: scabbard, dagger, lantern, cloak. There's something in your hand, you're using it for defence. Done.
Realistically, as long as you pay 10 gp and carry 6 lbs, you should be able to "skin" a shield as anything you want (unless your DM says no, I guess.) the difference is entirely cosmetic; therefore, no need for a feat.
Your probably right Stew, I had light damage/wield with a 2h weapon to bring it in line historically. But balance > historic accuracy. I think I'll keep the 1d4 bludgeoning though, I like the idea of a gladiator-style offhand buckler used to block and bash foes.
Why do you "have" to be a tank?