Coinbase has just announced that they are going to support ERC-20 assets, meaning that Ethereum tokens would be added to GDAX and later on possibly to Coinbase.
This means any of
these coins can get a listing on GDAX + COINBASE soon. Great news I'd say!

PS: what's your best guess for the first listing?
PPS: there's a framework for listing on GDAX, here's the
documentSource:
https://blog.coinbase.com/adding-erc20-support-to-coinbase-fe9cba6782b