Thunder Brother
God Learner
Doesn't a person already benefit from half cover when behind an ally?Kind of where I'm going with it...
Half Cover provides not only a +2 AC but also a +2 to Dex Saves. It would also allow a person with a shield to function as half-cover for allies behind their shield's facing.
So it'd still be only a +2 to AC, but it would increase dex saves and be more useful to a group.
Adding more shield types would be interesting though.
Buckler: +1 AC but can be doffed/donned as a free action. Accessible to rogues, for thematic reasons.
Heater shield: standard +2 AC
Tower shield: +3 AC but slows you down and has a STR requirement.