So this is the typical "You can only play one X forever" thing, but with the twist that instead of one game, I'm allowing you to pick one series.

Assume you get all DLC or whatever and whatever secondary things are necessary will exist, e.g. an Internet connection and a sufficient playerbase for an online game, all that stuff. You can use mods.

Now, defining exactly what is part of a series is a bit trickier for some games, but we'll basically go for anything that either has the same series in the title (So FF would include Tactics, but not Vagrant Story, for example) or which is explicitly in the same series even when games are unrelated or in different genres or whatever (System Shock/BioShock, Warcraft/WoW, etc.)

We'll not bother with projecting too far into the future, but if a game is announced and all that, assume it will come out.

Go.