I always pick the person to vote for in the primary based upon what I want from the leader. Richard Ojeda, for example is a great motivational speaker, he represents well on many other Democratic Party values but is terrible on economic, environmental and race issues. I'd vote for him if he won the Democratic primary but I wouldn't vote for him in the primary.
I don't really know enough about Gillum. It might be better to let him grow his political career and contend for higher offices rather than park him in the role of Veep.
Its all pretty early for me and I'm listening rather than deciding about the 2020 races.