How about it wasn't spotted on the ark and some how it goes to Earth and brings back Megatron Soundwave Starscream or Humans come back to the moon and find it and activate it.. Like in the wrong hands ect.. Good idea to bring back old Transformers that died in DOTM..
You know, there are seemingly new Transformers in DOTM that DIDN'T die that may make for more interesting stories than Megatron somehow getting his ravaged decapitated head reattached in some convoluted manner only to get knocked off again in 2 and 1/2 hour's time. I mean, he's Megatron... not The Black Knight.