The Zero Project is a ROM hack project that has been completed by Justin3009 with the help of many other users across a couple sites.
The project originally aimed just to have Zero be playable through bosses, but with further interest, the project developed into a full rom hack in which Zero was playable in every aspect. He has his own dialogue that alters the writing of the game to aim at him and many other pieces of dialogue have been altered to dictate 'two' hunters instead of just X.
A couple of features have been added on the side:
- Mega Man Zero styled sub-tanks in which it stops refilling your life when it's full.
- Zero's Z-Sabre has been nerfed to accomodate for the game changes and each boss has a variation of damage they take from it. However, if you decide to 'damage' him and give X his Z-Sabre, the Sabre wave still retains its full damage.
- Vertical Air Dash animation has been quickened.
- Zero's Z-Sabre animation has been quickened both ground and aerial.
- A pre-boss warning when fighting Mosquitus, the boss that damages Zero, in which the main menu loads and X warns Zero about the danager ahead. In that instant, you can switch out or keep playing as Zero and lose him as per normal.
- Zero has his classic 'Black Armor'.
Though this project is technically listed as complete, there is a version '3.0' in the works in which all of the code that was added in the original has been heavily altered and more organized, thus allowing more emulation compatability on other systems/devices.
It's been noted that X WILL have another armor and there WILL be a non-canon secret ending with at least 2 variations to it.