There is actual a class on the D&D Wiki, called THE SHIELD FIGHTER
that incorporates most of this into it already. Here is a Link to it: Shield Fighter
It takes on paths, allowing you to either attack from afar, boost damage, or maximize defense. These paths are, respectively, the Shield sling
path, the Great bash
path, and the Multishield
Each path has it's own strengths and weaknesses, but be warned that you should not allow certain skills in order to keep them from going munchkin with it. Specifically;
A)For the great bash path they should not be allowed powerful build, or they could hit for d12 by level 3. even MORE soon after.
B) Do not let MULTISHIELD path get any AC adding feats, or let them use the following feats from the homebrew on D&D wiki: Shield Edge Deflection, Shield Cover, Energy Deflection, Shield Focus, or Shield Harmony
. They are already bad enough as is with AC from both shields.
C) Honestly , this one you'll have to figure, out, as I have never made one or seen any other person use this beside Captain America himself.
As for anything else, I would say this class is GREAT in PHYSICAL DEFENSE, but has little to nothing against magic of any kind and is weak against touch attacks specifically. So id you have one in your group, plan ahead for the LIVING TANK.