Free Potential Story Ideas
Story-wise in regards to implementing crossovers, politics would make for a good story as the introduction of alien mercenaries and new technology into the already existant wars will make for a good writing. This will allow for a good bit of worldbuilding and tech crossover. plus you could use this to show the effects of dealing with a relatively stable group of alien polities on the fractured and warring humanity as they try to stop the war between the IMC and Militia.
Just bypass the first contact war stuff, every conceivable permutation of this thing has already been done and the amount of pro-humanwank in most versions would give the reader hairy palms in half a chapter. Come up with something new and have fun.
a slapstick comedy with gruesome robotic dismemberment and silly one liners.
An expeditionary group of several alien species from the Council races end up in the Yuma system during the refueling raid and get introduced to both warring factions and several members of the crew end up scattered and taken prisoner by both the Militia and IMC and experience the events of the TF1 Campaign.
A Pilot & Titan duo (Not Cooper & BT) find themselves transported into the ME Universe
Two Simulacrums end up in the ME universe and find themselves trapped in a galaxy that hates AI and will find themselves hunted and eventually recruited into Shepard's team
A Militia or IMC fleet (OR BOTH) appear above the skies of an alliance colony and are trapped in a new galaxy with aliens and humans whose technology are entirely different from theirs.
Two rival pilots belonging to either the IMC or Militia end up in the ME universe after their battle is interrupted by unknown means and are transported into the ME Universe and are arrested on the Citadel Presidium.