<p>yes they should</p>

yes they already shared information on official page. they told us they will add more coins, first they will add 5 coins from top 20 coinmarket coin list. and after 5 coin listing they will think to add more coin in upcoming months. 
and every exchange adds coins day by day bcz people want more coins for trading in single exchange, so they can buy all stuff in single place. if there is more demand that means more coin listing will happen. 
but coinbase is best exchange so i think they will trying to add only good project. and they will select only that project who have working product and scope of the project. if project have more use-case then coinbase will add more project wait and watch. 

Yes why not ? they are also an exchange and they already bought a decentralized exchange recently it looks like they want a bigger share of the market and the only way is to have more coins. they seem to be on the right path.