I've mentioned to people before about my idea for a crossover game with Commander Shepard and Master Chief saving the world together. They will be an unstoppable force of friendship, sweeping aside all adversity.
Then I had a better idea.
It'll be a crossover game where you play as Wrex and Arbiter.
For instance, at the end of an early level, Wrex and Arbi will be fighting against impossible odds, consigned to a competition of who will die the most heroic death. Then in an FMV cutscene Chief and Shepard will burst in, assault rifles blazing, riding on the legs of a Reaper painted in N7 colours. After mopping up the enemy forces, they will give you a curt, "good work soldier" before riding off again to do what they have to do. All of this will be entirely unforeshadowed and completely unexplained.
In the next level you will sometimes see the trail of destruction that the other heroes have wrought in the background environment. Sometimes looking at a key item will trigger a FMV flashback sequence of them doing something amazing, something that you were not a part of. By the end of the level you will find the entire husk of the reaper previously seen, wrecked in a fierce battle. As you climb to the top of the wreckage, you will see a large explosion in the distance and a Pelican fly out of the fireball. As an Easter egg, if you zoom in with the sniper scope, you can see the Chief and Shepard give each other fist bumps on the open back ramp.
The cutscene proceeding the penultimate level will have you unexpectedly finding Master Chief and Shepard floating in space unconsciously. When they come around, they will tell you about their fantastic adventures and how the world's forces are making a big mistake. Then they will tell you to turn back to rally the fleet and while they borrow a shuttle and continue on ahead.
The last level will have you leading the largest allied armada ever assembled in the history of the universe into battle, yet against an enemy far superior. But as you approach the battlespace, a tremendous explosion at the heart of the enemy force occurs and the entire operation turns into a mop-up action. Your mission becomes a search-and-rescue mission as you attempt to retrieve Shepard-Chief in the melee. You fight through the wreckage of an enemy vessel to their last known positions, behind a closed door. You open the door and you find...nothingness...a wound into space. They are gone: missing, presumed dead.
This will leave an opening for a sequel.
The promotional art for this game will feature Wrex and Arbi staring in awe in the foreground as Master Chief drives a flatbed Warthog off a cliff in the distance and Shepard is standing in the back holding two swords.
Imagine: you've crucially managed to open the gates to the reactor core just in time so the other heroes can go in (by jumping a Warthog across a chasm) and clean house.
The overall feeling you should experience at the end of the game is that you've had a strong secondary role in saving the universe.