Have a trading post that allows any player to get any card by ratio.
10 commons from the same set --> 1 common of your choice from the same set
7 uncommons from the same set --> 1 uncommon of your choice from the same set
5 rares from the same set --> 1 rare from the same set
3 epic and/or legendary cards from the same set --> 1 epic or legendary card from the same set.
1. This would create an effective way to value the trading ability of cards when players are gauging card value via trade.
2. This would create a decent card sink for extras or "low-end" cards of the same rarity...helping cards retain value over time.
3. This actually increases the value of "low value" cards given the ability to trade for a similar rarity through ratio.
4. Useful information could also be gathered on which cards are most often traded.
5. Being able to use the trading post in the same set prevents blasting away your extras to collect everything you need from a new set right away.
6. Ratio may feel steep, but finishing a deck or collection vs losing extras justifies the trade value.
They definitely should have something like this although I think what you get should be random, like if you trade in 10 commons you have a small chance to get an uncommon, but mostly you will get a common. Or say you trade in 50 commons, then you have a chance to get a rare/epic/legendary card.
There could also be some sort of "rule" that stated that you never would get the same card as any of the ones you trade in, adding a want to not simply trade in a lot of unlimited cards and avoiding the "I traded that card and nine others just to get another one of that card -_-".