They are probably going to add Cardano by the next time.
Forget about Ripple, they do not accept centralized coins, they wrote that on their ToS.
I'm pretty sure Coinbase is going to add coins by marketcap. So Coinbase is most likely going to go with Ripple, Cardano, or Dash.
Because IOTA is a real shit, it does not deserves to have iota on their platform.
Why not IOTA, you say? I think development-wise IOTA is the hardest to impement, as it doesn't use blockchain. Hope it gets added though.
And you are missing that, just like you said, IOTA is not using a blockchain, they are using another blockchain system but i do not remember the name of that chain at the moment, so no, they will never be added.