Only a presidential pardon will really suffice to come home without threat of arrest. It may very well happen posthumously, but that of course wouldn't be too useful to him by then.
A presidential pardon for Snowden won't come from the current occupant of the White House, and by the looks of the candidates so far he'll likely be waiting at least until 2020.
Are you sure? In their final days in office, lame-duck Presidents can (and do) use their pardon powers in ways that would be politically-unthinkable at any other time.
A total pardon would be very hard – a potentially dangerous precedent, even, for discipline in the security agencies.
But perhaps a pardon could be creatively crafted for all acts except some token offense with a smallish (2-4 year) maximum incarceration.
Without an unambiguous and expansive public pardon, if I were Snowden, I'd be somewhat suspicious of other offers from the DoJ. They might find a way to nullify the offer, or find something else outside the agreement to prosecute.