Summary
Barton had a career year in '17-'18, posting a career high 53.2 eFG%, as he finished really well at the rim and also shot 37% from three. He turned that into a 4-year, $54 million contract in the summer of 2018, which he can opt out of and become a free agent this summer. A year ago, I would have expected Barton to opt out, but I think it's a close call now. Barton missed some time near the end of the '19-'20 season, which was unfortunate for the Nuggets because they certainly could have used him in the playoffs.
 
Barton is a solid scorer who can buckets at all three levels, and can spotup or create shots for himself near the end of the shot clock when necessary. On defense, Barton's not a lockdown defender, but he's adequate. He's big enough to defend multiple positions, even if he's a little too small to defend the bigger, elite wings.
 
Barton does have some leverage over the Nuggets as they won't have cap space even if he leaves, so they may not have a great way to replace him. This is what John Hollinger deemed as the "Bird Rights curse" and why guys like Marcus Morris, Jordan Clarkson, and Joe Harris got so much money last season. Even if it pushes them temporarily into the luxury tax, the Nuggets could end up giving Barton a sizeable contract rather than watching him leave for nothing.
 
Cap Considerations
Barton has a $14.7 million player option for 2021-2022 that he might decline. If Barton does opt out, he will be an unrestricted free agent with a $20.6 million cap hold and full Bird rights. Since the Nuggets will be operating over the salary cap, they will retain Barton's Bird rights, and have no restrictions on re-signing him. However, the luxury tax could come into play, as they will be really close to the luxury tax threshold if Barton opts in or is re-signed, and that's even before re-signing Millsap or using their MLE.  
 
Therefore, other teams will have a legitimate shot at signing Barton away from Denver without having to make him a huge offer. Barton would most likely demand a starting salary of more than the full $9.5 million Non-Taxpayer Mid-Level Exception, which means only teams with cap space will be able to sign him. If Barton and an over-the-cap team have mutual interest, sign-and-trade options can be explored.