First idea - Prime Arcee gets stuck working with Armada Starscream.
Arcee is of course extremely pissed, but is surprised when she realizes this Starscream actually has a code and some measure of integrity.
Starscream meanwhile is shocked and insulted by hearing what a preening cowardly jackass this other version of himself is like.
They eventually grow a tight bond
And then Starscream heroically sacrifices himself again and Arcee's trauma gets even further fucked