I’d argue the President should not be allowed to issue pardons that are:
(1) Preëmptive (i.e. absent conviction);
(2) To himself, his current or former Cabinet members, or to any of the foregoing’s current or former spouses or children or grandchildren (or their spouses); or
(3) Issued after the presidential election in the final year of their term.
Furthermore, pardons for violent offences or corruption should be prohibited; provided, however, the President should retain the power to commute such sentences, and the Congress should have the power to regulate the manner in which the President may commute such sentences.
(Notably, I don’t believe this would apply to Ulbricht. He wasn’t convicted of a violent crime.)