Summary
Sumner has been an intriguing young player for a few years now, but hasn't put enough together to show he can really be a consistent rotational player on a good team. On offense Sumner is best in transition where he can use his speed and athleticism to get to the rim. He doesn't have a ton of craft finishing inside, which is why he's better in transition where he doesn't have to navigate finishing in crowded areas, but his overall shooting numbers a the rim are good.  He hit 41% from three this season, but the volume is low enough that you probably can't trust it, and he's only a 34% three point shooter for his career even with this hot year.
 
On defense, Sumner's slight frame makes it difficult for him to handle bigger wings, but he has good length and athleticism to be solid at that end. I don't expect Sumner to hit free agency, but if he does I wouldn't expect him to get more than a minimum contract.
 
Cap Considerations
Sumner has a $2.3 million team option for 2020-2021, and if that is declined he will be an unrestricted free agent with a $4.1 million cap hold and Full Bird rights, meaning the Pacers would have no restrictions on re-signing him.
 
If he becomes a free agent, other teams will have multiple ways to sign Sumner as even teams without cap space could possibly use any of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE or Bi-Annual Exception to sign him. There's also a chance that he ends up settling for the minimum.