Summary
Portis has a clear role, and that's as a scorer. Coming into the league as a stretch-four, Portis now mostly plays as a stretch-five. For those that aren't high on Portis, it's usually been due to his inefficiency on offense. In '19-'20 with the Knicks, Portis had a 50.3% eFG%, which was in the 18th percentile for his position, per Cleaning the Glass. However, Portis lit it up this season in the Bucks spacing, shooting 47% from three. I don't expect Portis to shoot that well going forward, but he certainly has the ability to shoot 40% from three, hitting in the mid-to-late 30s most of his career.  

While playing Portis at the five can create a lot of spacing and shot-making on offense, it can really hurt your defense. Portis was never a strong defender as a power forward, and at center those deficiencies became even greater, as having no rim protection can be a death sentence for a team's defense. Portis's teams have almost always been worse with him on the court than when he sits. After being benched in some early games, Portis was big for the Bucks in the later rounds, using his size to bully smaller players inside and on the offensive glass.
 
Cap Considerations
Portis has a $3.8 million player option for next season, and if he declines that option he will be an unrestricted free agent with a $4.3 million cap hold and Non-Bird Rights. If Portis opts for free agency, the Bucks will be somewhat limited in their means of re-signing him. Since they will only have his Non-Bird Rights, the most they could offer him using the Non-Bird Exception is a contract starting at $4.3 million.
 
The Bucks will likely have the $5.9 million Taxpayer Mid-Level Exception available to re-sign Portis if $4.3 million isn't enough. However, the Bucks could be well over the luxury tax if Jrue Holiday re-signs, so re-signing Portis could cost them a lot in luxury tax penalties.
 
Other teams will have multiple ways to sign Portis 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.