Yeah those are the kinds of negative interactions I knew I wouldn't be able to foresee.
Charged and Stack shields would definitely have to be excluded. Reduce I can't think of a reasonable accommodation to prevent it being abused with shield swapping. Maybe it could be treated as a charge/stack shield? e.g. "On play, become a random number of stacks from 0-3, reduce damage by number of stacks". Then additionally wouldn't get returned to hand as stack shield are excluded from the return to hand rule. Although I'd imagine that'd come with it's own stack of problems, obvious one being 0 damage reduce would have to be 0 stacks...
Same with playing shields to clear hand up, would be a significant change for some decks. Perhaps caveat that playing the same shield/weapon as you have equipped neatly removes it from your hand as usual? But that feels like a major new mechanic/rule to the game in addition to weapon/shield return to hand mechanic.
Another troublesome interaction I thought of is Wicked Shield, would it preserve reduction in hand? If so, would it tick down in hand? Lots of unforeseen considerations for what I initially thought wouldn't have been too major of a change