We aren't really meant to know who sent the Tet. In the film, Beech explains that the Tet is another alien race that's seeking to take Earth's resources for itself. The Tet could be a sentient being all on its own, built by another race somewhere in the known universe that, after eliminating its creators, travels to other worlds, eradicates the dominant native species, and hoards the resources for itself.
Sometimes the scariest monsters are the ones we can imagine ourselves.
It's possible that whomever or whatever built the Tet could send another one, but they would really have to think about how they would do it. Humanity has shown they're fairly resilient, and managed to take down the Tet and reclaim their planet, and that alone may cause wherever the Tet came from the rethink doing it again.