Mail in ballots would work. Yep, that would be good and I agree, time to get started with them. VP considerations, remember I'm a numbers guy. Warren wouldn't help Biden get to 270, neither would Harris. California and Massachusetts are going Democratic regardless. Although Warren would bring an ideological balance to the ticket. But not a regional one. You would also have two old fogies, 78 and 70 if I remember right. Harris is 55, in my opinion a better fit age wise.
Back to numbers, I'm leaving out men since Biden said he would choose a woman as VP. Klobuchar, Minnesota, regional balance, from Minnesota a state in which Hillary won by a single percentage point. Klobuchar would keep the swing state of Minnesota in the Democratic column. Baldwin, Wisconsin along with the Michigan governor, Whitmer, either one would bring those states Trump squeaked by in 2016 back into the Democratic column.
Although Stacey Abrams, Georgia, has the possibility of delivering Georgia to the Democrats, I like her. She lost the governorship by 55,000 vote in 2018. She lacks experience being the minority leaders in the Georgia state legislature. She's held no statewide or national office.
I'd be asking the question who would help me the most to get to 270. Then again, I'm not a partisan and not really ideological. So perhaps getting to 270 isn't the top priority among those faithful democratic loyalist.
Demings, Florida, congresswoman is another possiblity. But congress members do not the the ability to deliver a state like a senator or governor. Two governors are also possibilities, New Mexico, Grisham and Nevada, Masto, but they're going Democratic anyway. No help in getting to 270
This sounds very much like you think Kloubachar is his best bet.
From a non-numbers "tone" perspective, I think you are right - Kloubachar helps bring back a lot of the people put off by nuttery in the primary and any leftward-tics Biden has made to try to capture the Bernie People's grudging willingness to vote Blue.