Fear is the mind-killer...
Just finished watching it (and Children of Dune) again recently and while it is quite dated I still found them quite entertaining and can't picture anyone but Alec Newman as Paul Atreides. Not even the dark haired guy that fought Captain Picard in the (even older) movie. Still, the question stands though. With quite the lore, the unique setting, and I would guess fan base background behind the many incarnations of Dune (books, movie, mini series, RTS game, board/card games) you would think it would be prime MMO material.
Gameplay wise it is probably best to set it before Paul Atreides shows up for a few reasons, one of which being that he mainly curb stomps everyone once powered up and then his son does for 3,500 years (yikes)! So with that in mind lets see how it does in the other "usual" MMO categories.
Setting and Environmental WeatherSure, you can make all the extra home planets and / or space ships if you really want but I think you really just need to focus on the titular planet itself. Given it is mostly desert and rock that should even make it easier to render, leaving most of the designers to work on space ports, underground locales and seitches (villages). This means there's no rain, no snow, no clouds. Just need to make day and night cycles and the occasional sand storm.
MonstersMost of the bad guys in Dune come in the standard human shape, because they basically are all humans. This means it's easier to design shareable armor and clothes all around since your figures can share the same base models. The only other major critter is the sand worm. Sure you can make sand trout (the smaller ones) but those are meant to be rare. It's the gigantic sand worms that people really remember, and they are nigh unkillable through conventional means.
Mounts and Fast TravelOrnithopters serve as the usual light transport mode for those who can afford it (meaning catering for flight). Those skilled enough can also try ride the giant worms like in the books. Then of course you can throw in all the mechanical units from the RTS game at your leisure. General rule is the bigger the engine, the more vibration, the more chance of attracting them worms. Probably should also require spice to run. :P
WeaponsThis is a setting where there are lasguns, gatling guns, personal shields, and ordinance of all kinds. Yet most of the killing seems to happen with bone knives. Really depends how the game designers want to go here. I'd think that the easiest way to not go berserk is just to create a few weapons and have expensive ammo (or use the Gamma World method) for the ranged stuff which will really force players to decide how much they want to pull the trigger.
CurrencyTwo that I can think of. You have the spice. The whole reason everyone wants the one planet that produces it, and it is used in pretty much everything (crafting, healing, fuel, gaining powers). Then you have guild credits (or some such) which is what everyone that doesn't have spice uses. Which one is your pay for money currency depends on what you can buy with what.
Factions and PvPDune is all about PvP. Apart from the houses of Atreides, Harkonen, Corrino (and maybe Ordos for the RTS guys), there are also the Spacing Guild, the fremen, and the Bene Gesserit - all with their own agendas. NPCs of warring houses would obviously constitute the PvE side of things.
Anywho, that's my simple layout of what could be. I would certainly like to hear your opinions not just on Dune as an MMO, but on other works that have a strong enough identity to warrant one. Game of Thrones maybe? Also if you do know of a Dune MMO, please let me know! I'd like to try it out!