How about the following:
At the start you get 40 cards, 30 from a predesigned mono deck for your element, and 10 you can chose yourself.
You can trade you cards with other people from your team unlimited, as long as you and the person you are trading with are in the you own area.
To trade with other people, you have to be at the same hexagon.
Cards can be bought at cities, but only from the element where you are.
The other cities all offer 3 different colours, and the other cards.
This way you have to actively travel around the map to get all the cards you want.
When doing so, I think we might need an organiser with as only task to look if everybody does so. Maybe we can call him the retailer.
I would be happy to apply for the job, if needed.