Good Evening Vedrit.
I would like to point out that, for the people around here that do the serious work, "Game Concept" is NOT a quick "well i suggest a game where you can fight and defend earth. it will be fun yay!" kind of answer. What they meant (and have pointed out) is that you need to really think WHAT your game is actually going to be.
Starting BEFORE the plot (saving earth etc) you have to decide
The genre / Type . Yeah, MMORPG, but what kind? FPS, third person strategy, team control strategy, turn based fighting Final Fantasy style, puzzle solving? will it be PVP based, PVM based, or a mix. if so, how will it work?
Allright, you have a Genre. Now, how are you going to implement it? if FPS, then forget the turn based options, and you will have just a very large Quake 3 arena (bonus points on quake engine i guess?
). if turn based team control, then how will it work with the "massive" player numbers etc. (playing your turn then come back 4 hours later to play again isnt really fun)
Now that you have that, then you decide on the mechanics of your game. RPGs are basically a mix of Story + stats growth. so you have to figure out how will that work on your game in a balanced way (not that everyone is max level after 10 mins of plaing, but neither that you have to grind for years to get ONE level)
Then, what will be the player's options, control scheme, and in general "how the game will be played" ie. imagine yourself actually playing it. where do you click/use the keyboard etc.
Now that you have that, how will the GUI will be? it has to be "simple" enough, intuitive, have a clean and good look etc.
after all of that you can FINALLY worry about the story "you will fight as a soldier to save earth!". Story is OF COURSE important. But you need to actually have an idea of the kind of game before using a story on it. Ever had that feeling "Awesome story!!! the game sucked, but the story was great!!"?
You need to provide with ALL the details possible, so the programmers actually know what to program. Dont know programming yourself? its OK!, this process needs conceptual artists too.
In conclusion. All of this goes for the design of "almost a new game" yes, why? because as ive heard, UFO:AI
ALREADY has a multiplayer option.
In order to adapt/create/port it to a MMORPG you need to do some serious thinking. And not rely on vague ideas and hoping thats enough.
Game Creating, is a "serious bussiness" (yes, even our great old school classics had a lot of thinking put in them. a lot more than on the "new" ones, as it was said already.)
Final Note: First, Sorry for the Unholy Wall Of Text of Doom. Second, Ive thought of this concept a looong time ago. so if you need suggestions or so on this MMORPG adaptation, feel free to ask.