A federal pardon can be issued prior to the start of a legal case or inquiry, prior to any indictments being issued, for unspecified offenses, and prior to or after a conviction for a federal crime.
Trump could have dropped federal charges on Assange or issue a Federal pardon for future charges.
Trump could have dropped federal charges on Assange or issue a Federal pardon for future charges.
See the second part of my reply.
There is no legal pardon for future crimes.