Coinbase is opening up to new coins:
Since they are doing it open source, pull request can be submitted. Litecoin have already been added to Toshi.
Coblee says that the old coinbase backend was tailor made for Bitcoin, but coinbase will start using Toshi themselves. Hence all coins supported by Toshi, would be easier for coinbase to enable trading with.
Perhaps Peercoin could be added to Toshi. At least its worth I discussion I guess.