The story will split between two characters. D.va and Militia pilot Jonas Reyes, with one switching to the other between chapters and vice versa.
D.va's part of the story is that MEKA is a partner of the IMC to recruit and train new pilots for their titans. Until one night, after witnessing a battle between IMC and Militia titans, she stumbles upon a damaged Militia titan named BN-9591. After fixing BN, D.va ask it what is it doing here and BN explain that information is classified. D.va then asks where its pilot is, and BN explains that its pilot was scouting ahead before it was ambushed by the IMC earlier. BN also tells D.va not to trust the IMC. Suddenly, the two are discovered by an IMC patrol which leads to their location being compromised. Without any other choice, BN asks for D.va to establish a neural link with it and be its acting pilot, and with slight hesitation, D.va agrees. And after fending off an IMC titanfall, the two of them now must work together to contact the Militia for support, all the while fighting the IMC and mercenaries along the way.
Another half involves Militia pilot, Jonas Reyes who recently become part of a titan drop squad led by Captain Jack Cooper. Their mission has them which lead to Jonas being separated from his team. Jonas was then founded by Talon soldiers before being saved by Tracer before meeting the rest of the Overwatch team and reuniting with his titan, VT-1442. Jonas helps the team in finding out of a deal between Talon and the IMC all the while helping Jonas and VT reunite with their missing squad. Which leads to a IMC funded experimental science project led by Dr. Faye Oxton which the IMC are planning to turn into a weapon against the Militia. Now Jonas must reach Dr. Oxton with the help of his newfound allies, and stop the IMC from deploying the weapon, all the while being stalked and hunted by Sombra, Widowmaker, and especially Reaper along the way.
I also imagine the game to have a fixed health and armor system however here, health is divided into separate bars which represents a certain percentage and health regenerates up to that percentage until players find a health pack to replenish it. There can also be collectibles such as audio logs, and bios for characters and weapons, as well as upgrades throughout the level which includes for weapons, equipment, and health and armor. Titans can also be upgraded as well.