the problem these games have, i suppose, is that the original storyline wasnt written with an mmo in mind.
so yes, you could write one for lotr that runs alongside it, and has no interactions with anything from the books (since if it did, it would logically be written in books. otherwise its violating canon!), but then whats the point of it being an lotr mmo? if you want it to be accurate, you could never even kill a ringwraith, let alone go up against the witchking, because they all have predestined deaths.
i think in WoW its working out quite well, but thats because its going onto the end of an existing storyline, whereas the end of lotr was fairly definate, and you couldnt really set an mmo after the events of lotr. (ie. end of lotr = made it to mt doom, destroyed the ring = good guys won. end of WC3:TFT = arthas ascended the frozen throne, to reinforce the most powerful being on azeroth = bad guys winning.). Additionally, blizzard have been vague enough with the background of Azeroth that its fairly easy for them to add new things into it. (i dont think Ragnaros had even been mentioned before WoW, but now he's quite firmly an important part of the lore.). The same couldnt be done for lotr (at least, not if you want it to reflect the books) because the history of the world is so well detailed.