I cannot say at this time about Snowden but I do see Assange being subpoenaed to Congress in the future.

What better person to testify about what happened to Seth Rich?

I believe, if Snowden is asked to testify before Congress then it would be best if he was probably pardoned. One, you wouldn't want him under duress and expect to tell the truth and two, since he has a pardon he cannot Plead The 5th, therefore he is then bound to the truth or face being found in contempt of Congress.