Exp mastery or items.

Do you want to diversify the meta even more? Exp seems to be one of the biggest problems when it comes to duo lanes as both players will get a fraction of exp they would normally get while in a solo lane.

What to do? Offer some exp masteries or items to allow supports or non farmers to gain levels and keep up with the allies getting fed.

It would go something like this:

"Whenever a neutral monster or enemy minion is slain near both an ally and yourself, gain 30% bonus exp. This passive is disabled for 10 seconds if you kill a neutral monster or enemy minion"

The reasoning behind this is that champions scale in two ways: exp and gold. Supports aware hindered in both situations, leaving them weak throughout the game unless they somehow get fed. Their only real compensation are gold income items and runes to make up for their lack of scaling early game.

Implementing something like this would increase support variety and location, creating a dynamic system that could possibly end up in a unique match up every other game.