He's already a million miles too good (and rumour says he's only going up by 25 points anyway) - buffs your entire army, can output lord of skulls level damage output, can't be targeted if not closest and comes back to life in addition to having a 2+/3++. It'd be easy to tone down at least one of those options or boost his point to match said abilities. At the same time, it wouldn't be too hard to hand out some appropriate buffs to lacklustre SM units, either via abilities, a points drop, stat changes or stratagems.
Would seem like the simplest thing is just to either get rid of the reroll wound thing, or limit it to rolls of 1. And then at the same time just shave some points off every single Space Marine unit.

I am also thinking now though, that adding a few commander specific strategems would be a way to make it a little more viable to pick someone else than Grandpa Smurf.