I think people aren't thinking this thoroughly or rather go for a selfish choice that would seriously put a huge dampen on the gaming community. We would be talking about not being able to play anything differently except for what the franchise has to offer, and that is something that you have to really think through when trying to reply to this, not just your favourite franchise.

I'll talk about my case. Final Fantasy. Do I love Final Fantasy? Yes. Do I think it's the greatest franchise ever? No. Why Final Fantasy, then?

· Because it has over thirty games (quantity, so that people, at least, get a lot of games to choose from, if not play them all since there's nothing else).

· Because the formula is changed in every entry (meaning gameplay will be, in its majority, fresh between each entry, and there's something different in each case).

· Because there's a lot of spin-offs games that cover a multitude of genres (Shooters -Dirge of Cerberus-, Strategy -Tactics, Revenant Wings-, Racing -Bikes-, Rythm -Theathrythm-, the whole Chocobo games and the Tower Defense games -Crystal Defenders-, loot-based adventure games -Ring of Fates- and social multiplayer ones -FFXIV, Crystal Chronicles).

· Because there's something in every platform conceivable, meaning people would had multiple ways to get into this - only - gaming franchise.

· Because the merchandise from the franchise allows people of experiencing it in multiple ways than one: books, comics, anime, movies, figurines and all other kind of physical and digital items.

Now let's examine some people's answers:

- OP said Battlefield. Cannot stress just how bad this would be. Sure, FPS enthusiasts and FPS multiplayer gamers would have their time of the day, but what about singleplayer people? (Their only relieve would be Bad Company 1 and 2, which feature a solid and enjoyable campaign). What about people who don't enjoy FPS games? At least there's diversity having games set in almost every possible scenario, but this would alienate so many players it's not even funny.

· The Legend of Zelda. Now this franchise has had a long tradition and features many games. Quantity is assured, but on diversity lies the problem. We have two different versions: the isometrical, generally 2D games, and the 3D ones; both follow the same structure in most cases (There goes Wind Waker and Spirit Tracks to Combo Break player's traditional progression xD), and the gameplay is generally the same as well. There's some small side adventures like Hyrule Warriors for the Dynasty fan, and...eh...Link's Crossbow Training for those who want to go back to the on-rails day. I can think of one social adventure game (Four Swords, there's that Tri-Link game on 3DS as well), but the rest would be purely exclusive to a singleplayer affair that is, at its core, unchanged in most entries. While certainly a better choice than Battlefield for singleplayer gamers, this franchise would really alienate a lot of players as well, so it's not that good of a choice.

· Fallout. It sure gives hundreds and hundreds of hours, and while 1 and 2 are radically different to 3 and 4 (meaning there's that fresh sensation to experience), quantity isn't assured in this case. There are some side projects like that strategy game where you're a Brotherhood member (can't remember the name, Tactics it could be?), but you'd be mostly left to those four mentioned games. Mod support can take 3 and 4 very far, but to be fair here, unless you truly enjoy the core mechanics offered there isn't much here for most players, and no way for social gaming unless some miraculously multiplayer mod like the one in Just Cause 2 is added, which isn't happening anytime soon.

· Metal Gear Solid. This franchise would simply left aside a huge percentage of the gaming population. While a gaming favourite franchise, with a moderate/huge success in most cases, it would be hard for most people to get into some espionage games on which you tackle down huge mechanical monsters after the President of the United States touches your crotch to confirm you're a man. Just saying. Not to mention replayability isn't all that great in most cases, although there's a fair percent of social gaming here counting the spin-offs.

· Souls/Bloodborne. Talk about the epitome of alienation.

I think Mario would be another great franchise that cover most bases, if lacking any kind of massive multiplayer online, but still a much better choice than many others.

I thought about going with Super Mario for the rational reason that it has tons of quality games on a lot of genres, but I feel like it, similarly to FF, is a cheap answer to this question. Might as well just answer "The Elder Scrolls - with mods. Then modders can mod TES into all the other games that dissappear".

That'd be a terrible answer. Hardly anyone would revisit Arena and Dagerfall, so that only leaves Morrowind, Oblivion, Skyrim and TES Online. While the MMOPRG aspect is covered, ask yourself what kind of multiplayer option would people have playing the other three. Then ask yourself whether it is more desirable to have a football mod in Skyrim or playing Mario Striker's, when it comes to sport games. Or racing games, guess you could add cars in any TES games, despite being painfully out of place and probably won't work as well as a dedicated game/minigame?

Mods aren't the solution for everything. You can have a mod for everything, but that doesn't mean it will be fun or playable.