The most obvious route is to do a more or less straight sequel to Salvation, but the movie would have to be damn good to overcome audience apathy toward revisiting the post-apocalyptic landscape that just isn't all that suited to sustaining a full movie.
Yeah, nobody can relate to that future, especially when the characters weren't even all that engaging. It's best to leave the future as brief scenes like in T1, while relating to people in the present.
Right there, that's why the obvious route is a reboot or some more time travel. And if that weren't enough, you'd think they might take a formulaic approach by looking at other franchises that rebooted when they were in the shitter. Star Trek and Batman are two pretty good examples of success.
If all they do is continue where Salvation left off, they're just pissing more in that soup.