Hoooh, there's like a hundred of them, many of them I'm not really able to count, but If I had to pick one, my personal favourite might be the Main AU.
The plot for the main AU takes place after the events of both respective movies, where the FLDSMDFR is now the main core of the island, and The Fabrication Machine has long been destroyed in its own universe. Now, through multiple escapades, The FLDSMDFR gets stolen by someone, and through a little bit of tinkering, the person puts a chip onto the FLDSMDFR where once activated it causes the machine to gain a touchable, solid hologram human body. Alongside this, said mystery person also through unexplained events recreated The Fabrication Machine, also gave it a similiar treatment. Eventually, The Fabrication Machine made a run for it out of an angry and volatile response, dragging The FLDSMDFR along with itself. But unbeknownst to them, they've actually are faaar beyond their respective homes, even further than they would be able to reach. So now the two are stuck together, trying their best to accomplish their goals of reaching back into their worlds, and along the way meeting other robots akin to them, and also granting them with the similiar ability that they have (albeit the importance of the chip may vary between variants of this AU).