The loudest rumor is about Ripple XRP that would come to CoinBase wallets
Bitcoincash has placed itself to be a stable and useable alternative to bitcoin. Litecoin and Ethereum were too unavoidably popular.
But adding a dozens of coins would be counter-initiative for coinbase and Gdax. It would make it similar to any other exchange.
And I don't think coinbase would rush in that direction.
I don't see them dropping all the new coins as options at the same time. I think that they will spread them out and add them one by one. Each new coin that comes on can expect a massive pump in the days after listing.
There are still people trying to get into this thing that have a ton of money...
And seems to be a working strategy for coinbase. One of the most profitable strategy.
They bought a huge amount of bitcoincash before making the announcement. And they sold them at nearly a double price and with the fees.
So, adding a new coin will always be a Christmas to coinbase.