When in combat at least, each character should only have access to their own inventory. Maybe also to anyone they are adjacent to -- would allow for the whole 'Oh no, the cleric is down, quick, Ranger, grab the scroll of healing from them' type situation. But it would greatly increase the strategic management of your party's inventory.
Maybe as part of a difficulty setting? Or better yet a toggle separate from difficulty itself (or allow 'custom' difficulties)