The title says it all.. I know its enough to program a single game world... but could this game expand forever?.. especially with the server technology being utilised. releasing new worlds/planets and systems, and maybe even an entire new galaxy with hundreds of planets and systems the player can visit in there clans battle cruiser etc.. I don't expect a space combat system.. just a simple travel system with the ability to intercept enemy ships and therefore battle on the ship as an environment....

I have many more crazy ideas like this

I remember reading something about a new world after every couple months or so? Or until a faction conquers it. I am not sure though.

One has to start somewhere, then one can see what is possible to develop afterwards, suffice to say i do think we will be able to have planets for our selfs, faction based planets, space marine one, chaos one, and even perhaps design the worlds one self in the style of each faction.

Seems to me there is a lot of room for expansion

You and me both, Brother!
But yes, if the population stays high, or even continues to increase over the life of the game they very well could. Keep in mind the more you spread things out the more you player base will spread and the smaller your battles become.
Last i heard it was 3 month Campaigns on a single planet but each Campaign took part on a different contenient on the planet. So lets say there are 3 contenients on the planet, and they revist,twist, and tweek each one so you can play them... 3 times each without players getting bored. So 9 Campaignsm each 3 months long, thats 27 months for just the first planet! Little over two years for them to build the next world with new main features or races.

Personally i would like to see two eventual Campaigns, the first? Space hulk. Massive mashed together ships, still with huge areas and small passageways. Genestealers as the nid NPC threat. The second would be a Craftworld. This is the ultimate diveristy you can mash into a single place. Each dome with a different enviroment, 4 races battling it out on the Eldars home turff, could be awesome. I would like this to be far enough in the future of the release that Aircraft have already been released and they can add Voidcraft and Transports as your main method of travel to these two warzones. No Cruiser vs. Cruiser battles (though ive been waiting for a 40k space battle game) but still pretty epic.
I disagree with spreading it too far from a single contested battlezone though. This game is suppose to represnt 40k at its finest, big bloody battles. The strategy can still be a main factor but in the end they wont spread it out too far, i hope! At most i think something like Soulstorm would work, a bunch of smaller planets/ moons/ asteroids/ space stations(or ships?!) that have a couple of areas on each that can be caputred. Then you can still get the "multiple planets" type feel but really the map isnt much different or smaller from a contenient Campaign

Like those ideas - but I think if there were worlds , tyranid worlds for example, that can't be taken or owned, just battled on to reduce the numbers that attack your planet etc - and the size of game world, as you say, should be limited to allow proper battles.. but I think it can be big, and small together if the objectives move players to locations of battle (like a factory ). although without many NPCs around I don't know how the worlds wont seem sterile and derelict.

It would be like a cross between the continents on Planeside 2 and the mission select in DoW 2, where you choose a world, then a warzone. The problem is making these areas interesting, as making a whole world open would be impossible.

It appears that the events of this game will be canon. So it'll go down in 40k lore as a never ending conflict.