The ending of Transformers: Rise of the Beasts stops the Unicron threat for now: Optimus Prime destroys the Transwarp Key and collapses the portal, which strands Unicron instead of letting him reach Earth.
The key is the one object the whole film is chasing. Scourge wants it, Primal carries it, and Elena Wallace uncovers it. In my opinion, destroying it is the cleanest way to end that chase, because no later story can send the heroes after the same device. It could be argued that this also costs the Maximals a handy portal tool the series might have reused, but the film makes that trade on purpose.
The human beats are small and clear. Mirage has become an exo-suit for Noah, and Bumblebee has come back online, so the team that began the film scattered ends it whole. The Maximals and Autobots survive as allies. The Autobots make Earth their home, and Elena is credited for her find, which pays off her arc from museum intern to the person recognized for the discovery.
Then comes the last turn: Noah is recruited by G.I. Joe, which sets up a crossover between the Transformers and G.I. Joe franchises. I read it as a promise about where Noah goes next, not a resolved story, since the film does not show what the recruitment leads to. The untitled next Transformers film is in development with no cast or date, and TheWrap reports it is set in modern day; the new Transformers movie page tracks what is confirmed.
