Rights to what? They own the game and it's assets the European courts may have ruled in other situations about EULAs but until a case is brought before them regarding a mod to a game then you've got nothing to really work with.
The closest we've come to having a legal ruling on the matter was when Blizzard and Valve got into a huge legal fight over Dota 2. The Dota map editor IceFrog was hired by Valve to make Dota 2 but the original map creators of the map took legal action against that, the original creators being with Riot, which owns League of Legends.
Blizzard then turned to Riot and told them they were taking over, since like Bethesda they state in their EULA that they own the legal rights of their mods, or in this case custom maps. Riot gave it over to them instead of engaging in a legal battle over the matter and Blizzard and Valve faced each other. However the two giants settled on the matter but they did have to make some expensive concession to Blizzard.
If I made a mod I would automatically own the rights to anything of my own creation in said mod. Of course nobody else can have those rights as no matter what their eula says they can not take away those rights under European law. It is impossible to give them away (to be precise: you can sell the rights for someone to profit of your creation, but you'll always remain the original creator. Exception: any work-for-hire, but that doesn't apply here at all).
Here's the part most people can't comprehend. Creating something means you are the creator.
So lets say I create a porcupine-horse race. A Porcuorse, create the assets and stuff that thing into a mod. It does not matter if anyone stated anything in their eula, They don't own the porcuorse just because I made it into a mod that happens to run for their game.
Sure they own their game and their assets but not my assets , not my creation.
People need to stop seeing videogames as something new in this case it's just like say the electric windscreen wiper, cars existed (game), window wipers existed (races), but the electric window wiper for your car did not (porcuorse). And while it may have taken years in the end the car developer could not take away that the electric window wipers creator was the creator of said window wiper. Sure he used a car and even a window wiper, heck he even used a soldering kit and chips and whatnot. But it was a new creation in it's own right.
If it weren't a new creation and everyone had the rights to whatever someone invented based of their work well then honestly we wouldn't even have this discussion because clearly the ancient Greeks have all the rights to mods as there are only 7 plots in the world. Or maybe theunknown genius that invented writing (note: not real history!)?
So to summarize: If you made a mod for game then you do not automatically own the rights to it. I've love to see the day any mod creator goes head to head against Bethesda. As for any modder claiming ownership over any given mod versus another person, then you even in that situation you would still have to deal with the company that made the game.
Here's where it all falls apart, someone makes custom 3d hair models in a program not even connected to Skyrim. sets them up so other modders can use said hair models to make free content for everyone.
The hair models are the creation of that first modder, no matter what bethesda says at least in Europe you can show them the finger.
Someone takes said hair models and makes a paid mod using them. Here is the problem this person infringing on the rights of the original hair model creator to earn money.
The original hair creator can say you have no right to use my hair if you make money of of it.
Bethesda can say eh... well hmm... we don't want your hair. Maybe
However I can say screw you Bethesda I can change the data on my hard-drive all I want and still add the hair when I want.
One of my "friends' ( though he's a bloody prick ) is a popular map creator for Warcraft 3 custom game maps AND he's also in his last year studying law to become a state prosecutor in the US. I had a long discussion with him about the legal rights of maps/mods and he told me that it would very difficult for any modder to prove they own the rights of any mod they made for a game.
"...studying law to become a state prosecutor in the US..." - that's the part where I don't care about what he says.
Europe is kinda awesome, want to do business here abide by OUR laws, not the US laws. European laws/ maybe the country where you live if your country has even stricter ones than the European ones.
Now as far as maps inside a map making tool go, that is actually a lot more of a dark-grey area. Especially considering maps usually only re-use the game assets and are only a reinterpretation.
