After 1984 I imagine Hart is going to want safe as safe can get for his VP. Biden can bring blue collar bonafides, but he nearly died that year. So that's right out.
That aneurism could be butterflied away, but yeah, not the best choice when Hart would likely want to attack Bush for choosing Quayle.
Gore ran a fairly conservative campaign in 1988, but that's not necessarily a bad thing for that year. Young, decent experience, could help in the South. Certainly another person on the shortlist.
I think Hart would want to balance the ticket. Choosing a conservative would not bring left-wingers into the fold.