One could always make an Armored Spellcaster ability that half-casters have access to, that allows spell casting while armored or wielding a shield.

Druids are hosed by this, since presumably a Druid can not hang mistletoe from their shield and call it a focus, the way a cleric can with a painted representation of their holy symbol.

Yet if one does allow a Druid, to use their shield like a spell focus in the same fashion as a cleric or paladin, this would accomplish part of what you want.