Is it legal to make an open-source version of a closed-source game (and expansion pack) to distribute to other users? I want to do that for an old game called Age of Empires 2.
Age of Empires II can usually be run on Ubuntu and other GNU/Linux systems without too many problems, using Wine.
Disclosure: I am not a lawyer.
As for the general question, if your goal is to create free open-source software that facilitates playing a proprietary game, and you have programming skills, typically there is no legal impediment to writing a replacement game engine. (You may potentially fall afoul of software patents, but no more so than in any other major programming project to produce software for performing tasks that existing software already performs.) If you do this, then someone can install your engine, and then, if they have the artistic data (maps, "worlds", sounds, and so forth) from the game, they can play the game.
Developing a game engine for an existing game is a non-trivial task, and usually there will be differences between the way it behaves and the way the original game ran, though they might be slight. But the main thing you should realize is that the game data, representing the "world" of the game, will still be proprietary, and generally unlawful to redistribute. So people will still have to have a copy of the game (or at least certain files from it), to play it using your engine.
One example of a game where this was done is Jazz Jackrabbit, which can be played with the OpenJazz game engine.
Another example (where it was actually the original developer that released the GNU/Linux version of the game engine as free open source software) is DOOM (and since then, most of Id Software's game engines have eventually been released as FOSS, sometimes with slight modifications for patent reasons). There are a number of community projects that have enhanced the original DOOM engine, allowing for more complex maps, enhanced graphics, and a more three-dimensional gameplay experience.
Given the limitations and that writing a game engine from scratch (plus testing it and fixing bugs) is a significant task, you might decide to make your own game instead.
Disclaimer: This answer is for informational purposes only and not for the purpose of providing legal advice. You should contact an attorney to obtain advice with respect to any particular issue or problem.
