I don't really want to think about this. Unless Kyrie AND Al leave, we won't have any money to sign a reputable replacement. Not only are we well over the salary cap, but we are into luxury tax territory now.
If this were to happen, I assume we would build around Tatum and Brown (maybe Smart and Timelord, depending on his progression), try to consolidate our picks for another mid-lottery pick, and try not to be too veteran heavy. Al can stick around if he would like (he has an option) and Gordo doesn't really have a choice. Both are good veteran players who play well within a Stevens system, but they could also be traded for additional youth help. We wouldn't be tanking so their presence wouldn't really 'hurt' us.
Basically what I am saying is that if we lose our best player and try to replace him with a vet min/tax payer MLE type, we will not be better than we are now. It would be best to plan for the future at that point - even if it is the near future.