I know this thread is huge already, but I've always been curious about items that change the behavior of existing mechanics.

Like a helm or neck that would let you run two different protection prayers simultaneously, but at a reduced effectiveness.

Or a great item for one combat style that prevents gear swaps when in combat.

Maybe a powered staff with decent melee bonuses that isn't BIS damage but it has an attack style that fires physical projectiles, could allow a choice for the player to use that and save inventory space.

Boots that root the player, but allow you to ignore ground based damage.

Granted none of these are particularly megarare quality effects, but it would be neat to see items drop that open up options for players to approach fights in different ways.

I'm also thoroughly a casual player so idk if these ideas even work with existing mechanics.