Summary
It feels like we know what Len is at this point. Len appears to be a decent backup center, but nothing more. He has some skills on offense, and has even started to shoot threes (although that has tailed off a bit since his days in Atlanta). He's serviceable on defense, although nobody would call him an intimidating rim protector. Despite not having many clear weaknesses, Len also has yet to show he has a key NBA skill that will allow him to have a long NBA career.

Offensively, scoring more efficiently inside or hitting outside shots at a higher rate would go a long way into improving his value. However, Len is getting to the point in his career where you stop expecting significant jumps to be made. Len will be 28 this summer, so I don't expect age-related regression yet, but any improvements would certainly be unexpected. I would not be surprised if Len even had to settle for a minimum contract this summer.
 
Cap Considerations
Len will be an unrestricted free agent with a $1.7 million cap hold and Non-Bird Rights. This means that the highest starting salary the Wizards could offer him using the Non-Bird Exception is $2.9 million. This could be enough to retain Len. Alternatively, the Wizards could use their Mid-Level Exception or Bi-Annual Exception to re-sign Len if he demands more than $2.9 million.

Other teams will have multiple ways to sign Len 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 Len only gets minimum offers.