All the names mentioned in this thread are great suggestions and ultimately, if KD wanted out, the Nets would have their pick of whichever young star they want to build around alongside Simmons.
JB, Ingram, JJJ, even Herro+space. These guys already have big impact on the court and it isn’t inconceivable for them to develop even further if the team is built around them. In particular, JB and Ingram are at the top of that list IMO