Anyone with martial weapon proficiency should have no problems using the shield as an attack. I need to come up with some rules but giving people a bonus attack is a bit much I think. But if not why would anyone use a shield attack rather than the weapon?
So with Shield Expert you always get a bonus attack with your shield, without penalty? No, that doesn't sound good. (Or rather, it sounds too good.)- Shield Expert feat keeps the AC bonus if you attack with it.
So with Shield Expert you always get a bonus attack with your shield, without penalty? No, that doesn't sound good. (Or rather, it sounds too good.)

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.