Seriously, imo Warren's exploratory committee will determine that she has a good chance to be nominated -- politicians are all full of themselves like that -- and to be able to raise the money necessary. She'll announce. Sanders level of success in 2016 shows that someone with Warren's views has a good chance of getting the nomination. Whether or not Trump chooses to run, the Rs only shot is a strong economy. That's iffy and even boom times might not be enough for them to retain the White House or the Senate (Gerrymandering will probably assure the Ds hold and gain in the House). It's gonna take a lot more than Hiawatha jokes if Rs want to remain relevant.
The stakes in 2020 could be high. If Ginsburg passes and Trump appoints another justice, a D POTUS and Senate could very well choose to add 2 or 4 seats to the SCOTUS. D majorities in both houses could divide CA into 3 states, and possibly NY into 2 or 3, borders drawn is such a way as to assure D control of the Senate for all foreseeable time. If just a few of the right states get D majorities in their legislatures, the National Popular Vote Compact could be put into place, assuring D presidents for all foreseeable time. All this is straight from the mouth of a loyal leftist D, and it's not unthinkable.
Figure in demographic changes, and failure in 2020 could have Rs saying goodbye to not just power but to any relevance for a long time. They're going to have to up their game, starting now, if they care about that.