They could take some creative liberties with M3gan.
Gemma did have other inventions. There's the brawler bot, and the Furby knockoffs.
For those of you have seen the movie, it isn't out of the realm of possibility that M3gan could have created her own army towards the end.
There's also the variations on M3gan they could do. They could do the damaged M3gan that appears towards the end of the movie, there's a burned/charred M3gan, M3gan with no legs, etc, etc...
There's also some things that could have easily happened off-screen that are sort of hinted at/implied. I think it could stand as a house of they take some liberties (like in Army of Chucky) or somehow they're able to make it a mashup Chucky vs. M3gan house but I don't see that happening. Although I would really like to see that.
They could go the lame route and drop it in Terror Tram and just say that M3gan made copies of herself and connected them as a sort of hive-mind but I really hope they don't do that.