Something like Astral Shards but trad-able directly between characters(without having to go through an auction house). Preferably account-wide(just like shards) and dropping in-game with a frequency that makes grinding for them semi-reasonable.
The plat economy is dead. Item duplications killed the FRDS/LDS economies. The only economy left is Astral Shards, and requires the Shard Exchange to go through. We need a new currency to facilitate a direct player-to-player economy.