Inventory Slots besides Off-Hand

So there is the off-hand slot, but there are three more possible spots above and right know, they could be filled perfectly.

At head level: If you could place a telescope here, to use it with an key, it mixes the handy way of using from optifine and still isn't something available from second one

At chest level: Elytra, obviousle, there are even official Elytra x Chestplate pictures

At leg level: I'd love if this is a place to hold one bundle and any where else you cant hold them, like this, they arent overpowered.