This concept overlaps with a number of concepts that people have already submitted, but I think it still might be usable.
Concept: Innate Item
Description: A pokemon with an ability that imitates a held item, probably powered down. Role, typing, movepool, and stats would depend on which item is chosen. EDIT: The pokemon might get two abilities, each imitating a different item to give the pokemon different uses.
The item imitation might also be triggered by weather, but causing an effect that doesn't already exist.
(Using examples of existing abilities: Solar power = Special attack sunny life orb, Dry skin = Rainy double leftovers, snow cloak/sand veil = hail/sandstorm Brightpowder).
I see some issues with mechanics and balance: Just hypothetically, what if a pokemon imitating choice specs got equipped with another choice specs? Will the bonus be additive (2x special attack) or multiplicative (2.25x special attack)? Is that fair? Will that be disallowed by the ability? Will choice items be imitated at all?