Oh, wow. People still think of this bullshit? Please elaborate how is FNV is just 'a total conversion mod for 3'.
It is, it just doesn't mean that it's a bad thing. Same engine, a LOT of shared assets, a fair share of content like items, AI behaviours and entities are often 1:1. Hell, you can even find a lot of cut content from Fo3 if you tinker with it. You mention TTW, and that's another example of the games being so similar in a technical level that such a thing can be done.
Would you call any unrelated story taking place in the Middle Earth as the Lord of the Rings?
The Lord Of The Rings is a sequel to The Hobbit, takes place about 80 years later (or more), only shares a main character and about three secondary characters, and one's dead! Hobbit could have been a stand-alone story, as it very much was in intention. That is the case for most first installments/seasons, as you never know if you'll ever get a shot at finishing that overarching story.
Story and narrative wise, a sequel will be a new installment in the same universe, later in whatever timeline has been established. One that takes place before the events of the established events, but is released after, will be a prequel. They'll have in common also more things to be considered part of the same "canon"; characters, plot points, recurrent themes and elements, etcetera. They'll also usually belong all in the same medium, so you can't have X-1 a book, and then have X-2 a movie and be a proper sequel as a leap in the expression will count for a spinoff.
Now, we're talking games here, and as the expressive and interactive systems they are, all of the above elements can be translated to gameplay ones and still count. Perspective alone doesn't do it to call it a discontinuation. None of the diegetic and/or narrative elements might be there but an evolution in gameplay will still account for such, this being the vase for most more arcade or simplistic games. And of course, you can have both.
Fallout 3 is a spin-off because it takes place in a different canon, in a completely new system (mainly consoles), in a different perspective with different core gameplay, different effects on skills... And that's normal, as the reboot it was. You very well can consider them completely different games, and coming from Bethesda, we hardly could have expected something else in the first place.
In exchange, New Vegas IS a sequel because while keeping most of the design changes from Fo3, it makes some approach changes to better suit the original school of design, and most of all, it's set in roughly the same region, you meet either the remnants, stragglers and what have become of those original factions. It follows that story, and I don't know how can you really then call Fo2 a sequel as well considering that it has about the same connection to the previous as NV has to 2 itself. Fo1 and Fo2 having almost the exact same gameplay as well, only slightly more polished and advanced.
It technically could be called a sequel to Fo3, when only looking at it from the gameplay aspect.
Keep in mind, without the success of "muh turned-based combat" you wouldn't have a NV.
Without needing to wonder about parallel realities, that's the case with Fallout 3 as well, isn't it?
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