Trump should have the Feds charge Snowden. Then he should commute his sentence. So he’ll be able to return to the U.S. without having to worry about getting charged.
He was already indicted for espionage. So Trump could do it as soon as he gets in.
No matter what machinations Washington DC comes up with, he would be a complete fool to return to the United States. And he should not travel to any country where we could snatch him. Whether charges are dropped or not, we no longer operate under rule of law, but rule of whim. All it would take is a federal judge to rule the pardon illegal for one reason or another, and for the Supreme Court to refuse to take it up.
Currently in the United States, policy gets created, and the courts rubberstamp it.
He should just stay where he is and enjoy life.
Also, you do not have to be charged to be pardoned. They do not even have to name a specific crime. A pardon can be issued for a specific period of time.