Congress

Like LBJ being on the ticket with JFK to help bring the south, strongly into the D column. Carter chose Mondale to have a northerner on the ticket to help in states like MN, MI, NYS.
Reagan had Bush to help bring 'rockerfeller republicans' back into the fold.

But when Clinton chose Gore (Two southerners on one ticket!?! the horror!) the idea of a running mate for balance was disrupted. Both were young, considered to be moderates.

The last few elections cycles it mattered less (with GWB/Cheney, Obama/Biden, Orange cheeto/Pence).

This time around it might matter a bit more. Picking someone younger, perhaps someone black or Hispanic, might help put Sanders over the top.
Possible candidates?
Tammy Duckworth (if she is eliglble. Born in Thailand but to US citizens)
Julian Castro
Kamala Harris
Gretchen Whitmer