The only real issue i see lying in the future for GW is running out of ideas for new skills and professions, which i'm sure has been pointed out in several other posts already.
However, even if that does happen, it won't necessarily be the demise of GW. Even if new skill/professions become nearly impossible to implement w/o becoming too repetitive or throwing everything completely out of balance, they can still add new content like explorable areas, missions, stories, armors, weapons, etc; in which case it may be enough to entice pve'ers to buy an expansion only including that type of content. On the other hand, the large pvp population will take little interest in that type of content. In order to keep pvp'ers interest in new expansions at that point will be to add new competitive areas and forms of pvp. By that time, hopefully, there will be enough skills and professions to make the potential for new builds that are able to shift the metagame be virtually limitless, thus preserving the interest of that extremely important section of the GW population as well. Additionally, continuing to solve other forseen problems will be necessary, such as the player base becoming more divided. For that growing issue they created the customizable 'Hero' NPC characters that will make completing missions and quests much easier when forming a player based group is difficult or undesired.
Doing all this obviously will be no easy task, and I respect Anet a great deal for choosing this path and hope they are able to continue on it with GW. MMO's that are sustained by monthly fees, like WoW can afford to add a marginal amount of new content over time, because the game itself is set up with no real endgoal and people will keep paying to play. You simply pick an alliance, level to 60, get your uber gear and go wreak havoc on the other alliance, go on raid parties and the like. Engaging in those activities in that game maintains peoples' interest because the chaotic and random way it's set up makes it more for the sake of excitement and less strategy/goal oriented, outside of trying to increase your territory standing(and some dungeons do require team coordination and strategy). More organized pvp/pve environments like the one's offered in GW... as brilliant as the idea of it is, it doesn't maintain the appeal that an all out destruction fest does, because you're engaged in the same routine over and over.. and over without the thrill of victory that the "fast-paced, explosive killing" style of play has. For most people it starts to seem a lot less entertaining and less rewarding on an individual basis. Guildwars can't afford to keep adding miniature additions like sorrow's furnace because it's hardly profitable at all(which i'm sure they have realised by now), so i'm expecting to see few or none of those types of additions from now on, and see all new content to only be offered in full package purchase-only expansions.
In conclusion, I think it's very possible that Anet will be able to profit enough off GW long into the future. There may be some discrepencies along the way, like having no skill quests in factions, but what game is perfect? So.. until the face of gaming changes so much that any games made in this time period will look obsolete to new generations of gamers, I think GW will be ok. Until then, i wish Anet all the best with GW, and will always look forward to what they come up with next.
