It's a tricky point really.

If you use someone else's Universe as a theme for an RTW mod, it has to have certain elements. The idea of a D&D mod got little support simply because D&D was an adventure based game focusing on a small party wandering in the wilds chopping lumps out of anything that moved. It never really had any large scale combat EVER.

They invented Warhammer so the guys who had masses of lead miniatures for the game could have some wargaming rules loosely basedo n teh D&D universe ( a Warhammer mod is already underway for RTW by the way... )

Now...when you look at 'Book' based themes, again, you tie yourself COMPLETELY to the book by using it's universe as your base. I am doign a Mod based on Robert E Howard's 'Conan' books. This defines my basic world, gives me the factions and some of the units. It decides broadly speaking what should and shouldn't happen. If I tried to make the Aquilonians into 6 legged weasels, people would complain...and rightly so

If, on teh other hand, you want to make a completely fantasy mod based on astoryline of your own devising in a setting all your own, then that is possible too. I am doing one of those as well with my 'Robot' mod. There is no way people can tell me what can or can't be done here since I am defining the world/factions/background etc. and there are no sources for anyone else to quote

To use multiple sources of story, or to take elements from a variety of sources and try and merge them together is, I believe, a very dangerous thing to attempt. If you get the mix right, it could work very well, but if you get the mix wrong, then you will end up with a mess. You have to balance the whole thing up and fit stuff together for a reason You may think that Teletubbies fit OK into a Lord of the Rings scenario, but the purists would take some convincing....EEk...thats a rather scary image...A horde of axe-wielding Tinky-Winkies charging at you screaming 'ehhh-ohh' before they hack your troops to pieces...anyway...

To get to the rambling point eventually...you CAN make any mod you want. But..if you want to get an audience and have somebody else download it, either do the research and stick closely to your chosen literary inspiration, or start with a clean sheet of paper and invent your own. Trust me...fans of ANY particular book/Mythos/whatever DO NOT like seeing it messed with. If it's not something you know well, and something you are willing to research in depth...well.. you have been warned

If in doubt

a) Choose something obscure
b) Invent your own
c) Do it anyway and ignore anyone who doesn't like your version