Assuming the Genisys time line is only one. How can another Terminator movie be done without being stupid or a re-boot?
Uncertain. The Terminator franchise is on hold.
Given the mixed reception and underwhelming box office performance of Terminator: Genisys, it was announced by producer David Ellison that there are no longer are any plans to produce a sequel that ties up the loose ends in Genisys, and the Terminator franchise as a whole is now on hold.
Read these for instance:
- Terminator sequels on hold indefinitely; Oct 1, 2015
- Terminator franchise put on "indefinite hold" because of domestic box office performance; Oct 4, 2015
Given the negative reaction, it seems that the Genisys timeline has been abandoned, in contrast to your assumption. If the Terminator franchise is resurrected, it is very unlikely to pick up from where Genisys leaves off.
At the end of Genisys, we see that
John Connor survived.
He will inevitably attempt to rebuild Skynet. Judgment Day has once again only been delayed.
Edit: I may be wrong about exactly what survived, but something did. See this article.