I have actually daydreamed up a way to make a 40k MMO work,
though it would have to be different than alot of other MMOs to make it anything but elves in space.
There are classes, sorta, but in each of these there are different set ups, (battle brother, assault marine, etc) however they all lead to the same next story line, they just do it differently.
Space Marine: Must be earned rightfully, and is played like socom or halo. you are nearly invincible to gunfire, and can take massive punishment before falling. however, there arent alot of you (only PCs, not NPCs) so your going to be operating yourself a bit. However, only at the highest levels do you start getting squads of your own (veteran sergeant) and at the highest you get two squads under your control and your command squad (brother captain)
Guardsmen: The default human, you start off low, as a soldier following NPCs that do your quests for you (think guildwars with actual AI), then it eventually comes up to a mix of your badass Lt/Captain mixing it up in close combat with his advisors and commanding/strategizing on a HUD your available squads of guardsmen. However, to make it better on you, all the men in your command squad must die before you yourself can take fire, so it evens out with a marine player.
Inquisitor/Lord: A bit unique, you have the stat line (except for WS) as a gaurdsmen captain, while having access to things that even a space marine couldnt dream of. However unlike the guardsmen, you dont have alot under your control (usually), and must rely on your *Powerful* retinue, and your *very* powerful equipment to see you through. However, being an inquisitor you can call for different types of reinforcements, from gaurdsmen squads to actual space marine squads, to eventually terminators (on a long cooldown)
Chaos: I really havent come up with a good way to make this work yet, probably same premise as their loyal enemies but more....chaotic.
In the matter of PVP: it could probably be battlegrounds like in WoW, using your units and those of others on yourside, including space marines, though it would have to be balanced that no more than X amount of marines can be in one team. I like the idea of the awesomo meter mentioned earlier in the thread, as if you were charged as a guardsmen Captain by a squad of chaos marines, perhaps chance based, you could be inspired by the emperor to guide your blade true, and would go into a super duper luper close combat mode.
On Death: your characters minions should be murdered in very, very inhumane ways, but for your character himself, he should just be grievously wounded, and perhaps have a mini teleporter (like that Inquisitor from the Ciaphas Cain novels) and automatically teleport back to a hostpital ship, where you spend a good 30 seconds getting patched up before your launched back into the battle ground. Extra fun for drop pods, where you pick one place to drop and it scatters.
1) For character creation, I say at first only allow humans, and maybe xenos in the expansion. However, when you first start off, your a small child of 10-13 in your own instanced agri world, which would serve as not only a hub to tutorials, but lead on to the next part which is...
2) quest based character class: your class is determined not by whats on your selection screen but by what choices and quests you choose to do instead of killing bunny 23 and gaining a level. For example, you could be a 13 year old, and you have to go talk to the local PDF station to see if they need anything, while doing your duties your jumped by creature 7#, and while fighting it off you notice a robed man with the ultramarines U on his clothing, watching you (this isnt pointed out for you, you have to actually see it). Do you A) continue on and forget him (leads to being a gaurdsmen) or B) follow him and see what he could want (potentially lead to space marines)
Also, you could make it like how it has been suggested in the thread, that becoming a marine is insanely difficult, but balanced. How this would work is not only would becoming a marine be a skill based challenge (beat this training mode on legendary in under 4 minutes, twice) but it would fit in, as if you were a marine, you'd be just that, a marine. you could group up with other marines but thats about it.
Now for guardsmen, its a tad different. Its less skill based and more based around what your play style apparently is (you better at shooting, driving that vehicle, etc) and putting you in a squad of NPCs. Now for Progressing in this type of MMO, it couldnt work well with the standard level up system. It would probably go by ranks, and would follow sorta this type of progression:
1-10: your training days, you arent really anything yet, but can get a taste of everything before you decide.
11-20: (IG) your picked up, and put in your first real squad. Now the squad consists of 4 NPC pets, who'll follow you around town, but when it comes to quests once you accept they automatically move on their own and expect you to keep up. for (SM) this would be your preliminary tests, which would make or break if you keep going. by lvl 18 you should officially be in your carapace armor, However you are not assigned a squad.
21-30 (IG) your moving up the ranks, and you have proven yourself worthy of command, as such you are promoted to sergeant and get total command of a full 10 man squad, and to make this work your camera can zoom out, and you can move the squad as a group, and make them focus fire like how you would simply shoot with your mouse, though for heavy weapons I assume you could switch to an alternate fire mode and then shoot them, along with a long cooldown. Also, you start getting access to the armory, and now you can requisition weapons like power swords and hellguns, nothing terribly fancey.
(SM) You continue up your ranks, getting more implants. You now have the ability to join groups with other marines (not IG) and can now request different weapons from the armoury (bolt pistol, chain sword or bolter?) At 29, you take a final, large test which if failed too many times will lock your character from the chapter, thus either destroying him or putting him in the guard (albeit a buffed one).
31-40 (IG) you have gotten promoted once more, and are now lieutenant of your platoon. you are now placed in a command squad, and have 2- 10 man squads under your command. These you could command like you would a DoW guardsmen unit, and can set them to do different things. Also, your character has become more skilled than most guardsmen, setting himself above the rest. He now gets noticed by imperial scrutiny, and also by the forces of Chaos.
Here the poor Lt has to deal with a mini plot line, at first dealing with being kidnapped by Chaos, tortured unbelievably by cultists of Slaanesh and finally being 'rescued' (for lack of a lesser term) by inquisitorial stormtroopers accompanying an Inquisitor. The inquisitor talks to you a bit, and not matter what you say, you then have to quickly get away from the inquisitor before he and his retinue aggro you, and you procede into a big get away mission.
Once that is completed, you go through a plot line (which will lvl you from 31 to 38) to either clear your name, and in the end you confront the inquisitor in one on one combat. You defeat him, and are at a choice to either kill him, or be merciful and let him go, if you kill him, you either go back into the guard or fall to chaos, and if you let him live you are looked highly by inquisitorial agents or you go back into the guard. This eventually leads to a divergence at 40.
(SM) Not ones to be ignored, SMs also get a large mini plot to follow, one that I really havent been able to come up with, atleast one that doesnt sound too corny, However the end result is somewhat the same, you either A) choose to kill and fall the chaos (in this case a chaos marine/red Corsair) or return to your chapter, or B) forgive and get the chance to become one of the death watch, or return to your chapter.
40-50 (IG) your big chance, your really getting up in the ranks, and are soon called to talk to not only your regiment leader but the leader of the planet of your origin. They apparently have been talking highly of your skills, and have been wanting to promote you to Captain, however, theres one problem. Theres a hooded figure who wishes to see you and he has an Inquisitorial Signet, and when this happens people usually end up dead, so they want you to talk to him first, and if things are in the clear, then your on your road home.
So, you talk to him, and not only are you not about to be killed, but the Inquisition has taken a keen eye on you for not murdering one of their newer inquisitors, and they have seen your personal combat skills and are so far impressed. (of course you can turn this down and continue as a guardsmen) So insert more hand waving and you get the chance to become one of the three ordos of the inquisition, however, there is again a catch. you must take a test, not to test yourself physically, you have allready proven that, you must prove that you are loyal to the emperor beyond a doubt, so begins a long ish quest that determines your not only loyal, but if your going to be in the Malleus, Hereticus, or Xenos, depending on how you deal with your quest.
After this, you take a very interesting turn. You stop being in a command squad and now have your own retinue. You and your retinue take very tough, long, unique quests, but these end up with a boat load of XP. Not only this, but you can summon/call in squads of guardsmen and at higher levels a squad of space marines/terminators (inquisitor lords only).
Now its getting a bit late, and tomorrow Ill finish my guardsmen, marines, and chaos 40-50s, and other stuff.