Regret #1: Not pursuing John Collins more aggressively
The Spurs were reportedly interested in signing John Collins in restricted free agency before ultimately taking themselves out of contention with the DeMar DeRozan sign-and-trade. That proved to be a great trade for the Spurs, netting the team Thaddeus Young, Al Farouq-Aminu, and three picks.
However, by completing the trade, San Antonio used up most of their remaining cap space. Had they put off the trade and aggressively pursued Collins, they might have been able to negotiate a sign-and-trade with the Atlanta Hawks involving him instead.
It's unclear whether such a deal would have worked or not. However, considering the Spurs were reportedly open to placing Dejounte Murray and Lonnie Walker IV on the chopping block in a trade for Ben Simmons, that would have also been a good starting point in a potential trade for Collins. Add in a 2022 first-round pick, and the Hawks may have at least contemplated the offer considering their future luxury tax concerns.
Furthermore, having Clint Cappella and former sixth overall pick Onyeka Okongwu already on the roster may have pushed the team even farther toward moving Collins with a good offer on the table. Adding Murray to their roster would've upgraded Atlanta's backcourt defense and would've added another playmaker and scoring option on offense. Walker, on the other hand, would have provided the team with a young bench scorer and playmaker.
For the Spurs, Collins would have given them a significant upgrade at power forward (their weakest position) and is still only 23 years old. In reality, though, there’s no guarantee the sign-and-trade route was a viable option from the start, though it was one worth pursuing and wouldn’t have affected the DeRozan trade had the Spurs failed.