Does Dash ever have the plans to add proof of stake?
Masternode is POS...
It's proof of service, we shouldn't confuse him.
There is no proof of stake with DASH and I don't think there ever will be.
So then this masternode, is not creating proof hashes from coin outpoints?
Usually MNs just mix funds, not that they ever actually touch the funds, but that's another subject. When MNs process instantX transactions, they do so in a quorum of, I think, 10 MNs. They check the validity of the inputs, and then lock them. Once locked, they are included in the block by the miner.
In the future, there will be only instantX transactions. The MN network will all be split into quorums and they will process all transactions this way, taking all responsibility from the miners. If the miner tries to include something that hasn't been locked by the MNs then the block is rejected.
So anyway, POS, Proof Of Service is where the MNs each have to ping every so many minutes to prove they're online and ready to serve. If they go off line, they lose their place in the payment list, although they have 70 minutes or so, which is more than enough for updating and maintenance.
Miners are still important, because the hash they create is used to select the quorums based on their original transaction ID and their age. They are also tasked with updating the blockchain.