Coinbase says they are going to add a bunch of altcoins in 2018. Let's speculate on what they might be thinking of adding.
Chances are they will NOT be adding any privacy coins. See the statement issued the other day by the SEC Chairman. They will be too afraid of running afoul of AML/KYC rules and suffering a regulatory buttfucking.
Coinbase, meanwhile, is a YC company. So are Quantstamp (QSP) and Request Network (REQ). Both, however are priced in the pennies, which optically I think works less well in their favor. So maybe those are out too.
Many people have suspected that Ripple could be an addition. But again, it's a penny coin--despite having an $18 billion USD market cap (kek)
Bitcoin Cash is another possibility, but would be very confusing to normies seeing it alongside regular Bitcoin.