Increase Attack Energy costs calculated from class, strenght, dexterity, skill in shields, talent in shields, weight of shield(s). Archery as well, because the PC is un-equiping melee weapon(s) and shield(s) and equiping missile weapon and this takes 'time'.
Un-equiping weapons(even no-weapons) only, is already factored into the current system. A shield(s) are bulkier so more 'time' is needed.
Increase the Energy cost for casting hand gesture spells while wielding shield(s). More so for duel-wielding shields. Reduces somewhat by strenght.
Remove PV from non-artifact shields.
Decrease DV with DV gained from shield(s) while:
equiping, reading, eating, dipping, alchemy, bridge buiding, climbing, cooking, detect traps, disarming traps, lock picking, first aid, gardening, rubbing, squeezing, healing others, picking up, dropping, mining, swimming, smithing, shooting, throwing, playing, pickpocketing, mindcrafting(excluding mental shield), necromancy, dead scrificing, casting spells that require hand gestures( bolt, ball, burning hands, healing, curing, anti-poison, bless, destroy undead, etc. I think it's the majority). List is not complete.
But not from:
walking, drinking, spitting, praying, live sacrificing, ventriloquisting, melee attacking, standing, kicking, switching position, giving commands, chatting, haggling, open/close door, pull/push lever, casting spells that don't require hand gestures( Strenght of Atlas, Wish, Darkness, Light, etc. ) List is incomplete.
Edit - Ok, so I had some time to think at work. Some of my suggestions is directly trying to discourage magic-users to not use shields( maybe magic-users should get bonusses from wielding a stave ), and they make more realistic sense. ( I'm not suggesting ADOM must be realistic, just fantasy realistic. )