I am a member of the team modmakers. After a few simple modifications, we decided to go for something more global. This "global" was the addon "Wild West". It was planned that it will be some sort of DLC at the beginning of the Mojave Wasteland. But later it grew and evolved into a full game in the universe Fallout, with many modifications and resources F3 and NV.
So the question is: Is is legal to create such a "game" that will include resources games but not require them?
The most remarkable thing here is that will be implemented, the classic system of travel.

No, Bethesda does not allow you to mix the content of separate games. Their explanation is that they have licensed textures/models/etc. for one game and one game only, so reusing them elsewhere is not OK.

Unless you can verify you own both games in an installer for the mod (i.e. extracting the content from your own copies)

Even in that case Bethesda doesn't allow it. There was a bunch of drama a while ago about remaking Morrowind using the Oblivion engine, reusing assets from Morrowind via an automated conversion tool. Despite the fact that they weren't distributing any cross-game assets, and users were just running a program to extract/convert the data, the official statement was that due to textures/assets only licensed for use in one game, reusing them in another game was against the rules.

Illegal.

It's perfectly legal if it is similar to Requiem for a Capital Wasteland, in that you must own both games for the resources to work at all. If not, and your download includes those assets, then it is not.

Flat statements like 'illegal' aren't really helpful, as there's always debate over things like the legality of EULAs; it's more useful to explain the issue fully like ianPatt has, to avoid "But they're files on my computer so I can do anything and it's legal" when the issue isn't that simple.

E: ^^^ Is there a Requiem thread on the gamesas forums? Morroblivion got squished and it had one here, it may be that they don't have to act to show they're not encouraging breach of license unless the breaches are being advertised on their own forums.

I think that may be the important part here.
You could understand this statement in a way that your game uses resources from the games, but not require you to own (both of?) them.
In this case I'm pretty sure that the answer is no, it's not legal.