Cory Booker might be an option...He's relatively young, has had a fairly high-profile from his appearances on the judiciary committee, and is seen by many as a moderate.
The main complaint against him might be the fact that he's unmarried, which will give the far-right to attack him as being gay, or a pervert, or whatever. (Not that the attacks would be fair, just that they might be effective given a certain portion of the electorate.)
And he's not very experienced. I think that was a big part of Obama's problem. He ought to have waited another decade before running.
Booker first got into the senate in 2013. If he runs for president in 2020, he will have served as a senator for ~7 years. At least to me that sounds like a pretty decent length of time to figure out how the federal government runs. (And in that time he served on both the foreign relations and judiciary committees, both high profile and rather important.) Plus, he has experience as a mayor in New Jersey (sort of a semi-Executive position.) I certainly feel he has enough experience, and don't really know how much more benefit he would get from hanging around Washington for another decade or so.
Plus, there are drawbacks of waiting. The longer you are around, the more chance that you will either be associated with scandals (deservedly or not), and/or will be seen as a Washington flack.
if the Democrats want to ensure they get rid of Trump they need a centrist white guy - preferably ex-military.
You may be right in that the Democrats may need to return to a more 'traditional' candidate to secure victory. But, elections are rather hard to predict. Another minority running for the democrats might energize the republican racists, but it may also entice black voters (who voted for Obama but often sat out the Clinton election) to return to the polls.
That's the thing about racism... Trump managed to use bigotry to his advantage, even though his opponent wasn't a minority herself. He doesn't have to run against a minority to whip up racist sentiment, he just has to use the appropriate dog whistle. "They won't protect our children from the evil foreigners".