But also I imagine a few younger liberals will capture the attention of the Democratic Party, while Warren will focus on being a Senator.
By "early frontrunner", I mean 2017. If Clinton loses next year, who will be regarded as the early frontrunner for the 2020 Dem. nomination as of two years from now, in early 2017? I don't see much of an opportunity for any "younger liberals" to capture the attention of the Democratic Party between now and then except for Clinton's running mate. And as I said, there's no guarantee that that person is going to catch fire.
An obvious opportunity for someone else to catch fire would be if they ran in the presidential primaries next year, but it looks like Clinton is only going to get token opposition from Sanders and Webb, who are both older than she is.
I'd be kind of surprised if Kamala Harris doesn't get a plum speaking slot at the convention. But if Hillary loses, I think we'll see 2020 polls within a few days and Warren will lead them.
I also think even at 71 and with a GOP incumbent, she'd be more likely to run than she is now.