Transformers

Short Answer: 1) the 1800s and/or 2) "Voltron" the McGuffins 3)Monsterverse crossover

The Transformers movies seem to want to also be National Treasure/Da Vinci Code movies, rewriting actual human history to attribute events to the Transformers and their foes. 

But they kind of start that... and drop it. There is stuff about Camelot, and stuff about WWII. And... that's it. Still, having this idea gives us the possibility of genre crossovers, like the sci-fi/fantasy and sci-fi/ war movies so far. But there is a whole lot of history left to look at.

So, let's look. I recently learned that for part of the 1800s, the following four cultures co-existed in different parts of the world-- The Wild West (US), The Victorian Era (England), The Samurai era (Japan), and the tail end of the Pirate era (Caribbean). 

Which means we can have one or more of these genre crossovers. We're talking Transformers and/or cowboys and/or samurai and/or pirates and/or steampunk. How much fun would the CGI staff with steampunk Transformers? That turn into steam locomotives, steamboats, airships...

Plus, Charles Darwin, Sherlock Holmes, Dr. Jekyll...

...or Ebenezer Scrooge. Yup-- we could have A TRANSFORMERS CHRISTMAS, with TIME-TRAVELING ROBOT GHOSTS. If you think that could be cool...

And of course, still plenty of (violent) history left to shove Transformers into: The Revolutionary War, for example. Ancient Rome or Egypt. Napoleon's Europe. (Just rifle through the Drunk History or Epic Rap Battles of History playlists. History's Greatest Hits are pretty readily available.)

Another running feature of the Transformers series is high-tech McGuffins-- objects whose having (or preventing from having) drives the plot. These include the All Spark, the Talisman, the Space Bridge and Quintessa's staff.

But what if we borrowed a page from another classic robot animation-- Voltron. The gimmick there was to combine huge robots into a gigantic one. Now that I think of it, this is also done with the Infinity Gauntlet in the MCU.  

So... do that with the Transformers McGuffins. Join them into one giant MegaMcGuffin! What happens when they all join together. Whatever that whatsis is, is going to be pretty powerful, and everyone is going to want it.

Another option is to have the Dinobots fight MechaGodzilla, and so bring Godzilla, King Kong, and the rest into the Transformers-verse. Yes, Pacific Rim did this, but that was mechs (with people inside) fighting kaiju. Self-driving robots fighting kaiju might be different enough to work.