As we have seen with LiteCoin whatever coin is added to coinbase instantly boost up price. This happens because once added to coinbase a cryptocurrency can be easily purchased with fiat and becomes easily accessible to the masses. Even with all of its problems as far as exchanges are concerned coinbase is king, and is only getting better. Coinbase will inevitably add more coins because more coins = more money. $$$
So what would you like to see?
Bitcoin Cash?
IOTA?
Ripple? (XRP) Currently no micro transaction coin on the website. For newbies XRP would be very popular. .25$ coin vs 18,000$ coin, people like the idea of having 100XRP vs. .00001BTC.
Currently no privacy coin so perhaps Monero or DASH? I find this to be unlikely, but possible! Ever since Coinbase lost their battle with the IRS I do not think these coins fit their current guidelines.
Lots of possibilities!
Let me know what you think!