Here's what's up with Bumblebee's ending.
While Bumblebee may not have a proper post credits sequence, it does tack on an extra little bit of story before the credits actually roll, rounding out the plot and setting up for sequels--maybe? Bumblebee's stinger (pun intended) doesn't have a lot of meat to it, but it does start treading some strangely murky water when you actually look at it in the scope of the whole live action Transformers franchise, which Bumblebee appears to have rebooted--emphasis on "appears to."
Needless to say, spoilers to follow--not just from Bumblebee, but from whole swaths of the Transformers franchise.
Bumblebee highlights the first major Earth-based interaction of Bumblebee the Autobot, and a human girl named Charlie who finds him as a VW Beetle in a junkyard. The whole story is set in the 1980s, not long after the "fall" of Cybertron to the Decepticon army. Charlie and Bee are chased around Earth for a while by two new Decepticons, Shatter and Dropkick, as well as John Cena, before eventually saving the day and--tragically--going their separate ways. Charlie gets a new car--a refurbished classic Camaro she and her late dad had been working on--and Bee, having effectively blown his VW Bug cover, takes a new alternate form of a black and yellow modern Camaro.
Then, following their bittersweet goodbye, we see Bee rendezvousing with a recently arrived Optimus Prime, who hopefully directs his attention to the sky where around seven Cybertronian crafts can be seen entering the atmosphere. It's all very obviously working to set up the next movie in the sequence--though it seems to simultaneously be working to slot itself into the groundwork laid by the first Michael Bay movie back in 2007. It was that first movie in which Bumblebee chose a modern Camaro as his alt-mode, after all, and the place where the whole idea of Bee using his radio to talk was first introduced. If this movie does anything, it's provide Bee a worthy origin story expressly for that 2007 debut, filling in all the gaps that any fan would ever want filled as charmingly as possible.
However, filling those gaps and answering those questions also creates new ones. The Bay arm of the Transformers franchise began getting seriously off the rails with its own history right off the bat by establishing the presence of the "AllSpark" (a mythological Cybertronian artifact) on Earth as early as the 1800s in the first film. Later, in the most recent installment of the Bay movies, Transformers: The Last Knight, it was implied that Cybertronians had reached Earth back in the days of Arthurian legend. Prior to that, Transformers: Revenge Of The Fallen raised the idea that Cybertronians had been aware of Earth since antiquity and even been involved in the rise of Ancient Egypt.
But Bumblebee gives us a look at not only the fall of Cybertron itself--happening, apparently, in relative real time to the mid '80s on Earth--but also explicitly tells us that this moment was actually the first time Optimus Prime had ever scanned for or sent anyone to Earth at all, making Bumblebee the first Cybertronian to set foot on the planet. This later seems to be supported by the arrival of the movie's main antagonists, Decepticons Dropkick and Shatter, who make the whole "first contact" schtick an ongoing gag.
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Of course, this doesn't necessarily mean that future installments in the post-Bumblebee Transformers franchise are going to have to work from the ground up. Everything regarding the timeline is just vague enough that, with a little narrative hopscotching, it's possible to slot stories into the mix that justify the temporal strangeness. After all, if we're to assume that the AllSpark just hasn't been "activated" yet in the '80s, it's completely possible than the Transformers arrived on the planet and were none the wiser, left, and then came back when it was. The characters that would, in theory, be duplicates of robots who have already shown up in the Bay movies--like Starscream and Ravage--could easily just be seen here in older alt-modes or less advanced forms. There's no real limit to the amount of hand waving that can happen in a universe like this.
Or, more likely, future Transformers movies are just going to cherry pick exactly what they want from the established canon and easily forego the rest. Bumblebee may not represent a direct, hard reboot of the entire franchise, but it's certainly a major departure in tone and direction--while it may be making a vague attempt to at least loosely hold things together, it's absolutely not beholden to weave the entire shared universe into one cohesive thing. And to be honest, that's probably for the best.