No Transformers Movies Are In Development Right Now
As it stands, there are currently no Transformers movies in known development right now. There are likely moves going on behind-the-scenes at Hasbro and Paramount, but no official announcements of projects, directors or even hints at direction have been made. It was likely that Paramount was waiting on the reaction to Bumblebee before going ahead, and the numbers for that film are so complex it's hard to draw conclusions. It's clear there's some apathy towards the franchise both in the US and overseas, and this youth-spun adventure worked as a testing ground for the brand and ideas.
That's not to say there aren't plans in place. Movie studios plan years - sometimes decades - in advance, and given that Transformers is one of Paramount's prime properties, there will be a lot of ideas floating around. Producer Lorenzo di Bonaventura has said another mainline series film is in development, and more is sure to come. For now, though, it's unlikely we'll get a new movie until 2021 at the earliest.
Transformers Will Likely Reboot In Some Form
Whatever movies do come, it seems quite likely that the future of Transformers movies involves a more overt reboot of the series in some form. Bumblebee was a stepping stone for this, and given the positive response comes across in spite of what came before, that direction is clearly the route to success; any financial complications come from the Bay backlash rather than anything representative of the movie itself.
The type of reboot is up for question. It would be very easy to continue slowly diverging from the past, telling stories that aren't explicitly in or out of continuity with what came before. However, if Bay-phobia was a factor for Bumblebee, it may be more recommendable to cut losses and have a complete restart. That would mean Bumblebee has no narrative role in the future, although its standalone nature means that doesn't leave too much hanging.
Hasbro Wants To Build A New Shared Universe
All of this Transformers movie talk ignores that it's just one aisle of the Hasbro Studios toystore. Films for Dungeons & Dragons, Magic: The Gathering, Play-Doh, Monopoly, Clue and Furby are all in various stages of development, although that's nothing on a proposed shared universe plan.
Also in the works are G.I.: Joe Ever Vigilant (a continuation of the 2010s film franchise), Snake Eyes, Micronauts, M.A.S.K., Rom the Spaceknight, Action Man, and the recently acquired Power Rangers. The assumption is that Hasbro is trying to build a shared universe similar to Marvel Studios' unprecedented MCU (their Revolution Universe comics team up these very toy brands in an Avengers-style crossover); the script for G.I. Joe 3 reportedly at one point even included a Transformers crossover. Word on any of the projects has been rather slow recently, with the only dated movie being Micronauts in October 2020.
What Transformers' delayed expansion means for all this depends on how fundamental they are to the Revolution Universe. It has been speculated that John Cena's Agent Burns in Bumblebee is an original G.I. Joe or a member of M.A.S.K., but the film avoids providing any confirmation either way (likely to keep the door open). Even if that's a red herring, any delay on a form of reboot will surely push plans back further than Micronauts launch (as will G.I. Joe's movie being a third entry rather than a do-over).
Related: Why G.I. Joe Needs the Transformers
The future of the Transformers franchise may currently be rather murky, but it's clear that Transformers: The Last Knight was far from a final nail, and that Bumblebee's low box office will do nothing to stop the Autobots... or the Decepticons.