They only have that three coins.

BTC - ETH - LTC

Also don't think they will add more. I believe they want to appear to be the most regulated and stable ones.
So they will not want to enter in "dangerous" waters like other ones.

Who knows... it's my opinion from what i see lately.

They had a big pump of money somewhere in august this year:

The round's completion raises Coinbase's collective funding to date to more than $217 million, higher than any other industry firm that has raised money from accredited investors.

source: $100 Million: Coinbase Raises Biggest-Ever Round for Bitcoin Startup - CoinDesk

So... they have lot of money and don't think they care to go beyond that trio if they want to be there in the long term.

But in crypto world, who knows for sure what will be next in this moments.

Maybe after few more years will have a start for some quiet waters and a more stable market not as volatile as today.