I don't think Gabbard has much chance. She is from an insignificant state and there is likely too much bitterness left over because of her support for Sanders. Of the others mentioned, Collins and Warren are too old: on election day of 2020, Collins will be one month shy of 68 and Warren will be 71. The one with the best shot is Harris. She will have 4 years as a Senator (2 more than Obama had in 2008) and has a history of electoral success, having been elected as the District Attorney of San Francisco and also to the office of Attorney General in California. And if the Democrats are still going with identity politics she is a "threefor" : a woman, an African-American, and an Asian (her mother is from India).