Monero just recently deployed multi-sig support out onto mainnet. Which means multi-sig hasn't been battle tested on the mainnet. This reason along with potential AML road blocks that Coinbase could run into leads me to believe Ripple and Dash have a higher chance of getting onto Coinbase first.

Dash does have it's foundation behind it to help fund the integration development needed to list the coin. Brian Armstrong did say that if anyone really wanted to see new coins on the exchange then they should lead the charge in developing it.