Snowden will come back only if there is an attitude change in the USA resulting in a change in the political party in power being more concerned about individual rights, particularly privacy.
Right now the Democrats and Republicans are both split on the issue of privacy and rights versus big government survelliance. Given that situation what should happen is Snowden is pardoned by a US President - this would shield him from any future prosecution.
However Obama is not going to do that, he is a big government fascist - he'll pardon on his last day of office all the criminals that worked with him.
The vast majority of the American people do not understand Snowden or the issues at stake and driving his actions.