If ANet keep going at their current rate:
Prophecies: 6 professions, 30 profession combinations (6 x 5), 75 skills/profession
Factions: 8 professions, 56 profession combinations (8 x 7), 100 skills/profession
Nightfall: 10 professions, 90 profession combinations (10 x 9), 125 skills/profession
Campaign 4 (Spring 2007): 12 professions, 121 profession combinations, 150 skills/profession
Campaign 5 (Fall 2007): 14 professions, 182 profession combinations, 175 skills/profession
...
Campaign 10 (Spring 2010): 24 professions, 552 profession combinations, 300 skills/profession
...
Campaign 100 (Spring 2055): 204 professions, 41,412 profession combinations, 2550 skills/profession. j/k, j/k.
Okay so I'm not saying there will be 100 campaigns. But I think you can see where I'm going here. If you have bought every single campaign, like I know most people here on Guru will, imagine trying to choose from a hundred possibilities on what professions to make your character. Imagine trying to acquire over 300 skills (both professions), capping like 100 elites, with ANet's new idea of no skill quests at all! And that's just in campaign 4! The way ANet is going, even Campaign 10 doesn't seem that far off.
Here's my point:
How can ANet simply keep adding campaigns without
a) losing diversity and becoming repetitive - skills, items, everything will start to seem identical to old stuff. Already tons of skills are just repeats. Most item skins are worthless. The more different "rare" skins they add, the more the market is divided.
b) losing Class specialisation - just how many classes can you have that truly offer something different to a party? There can only be so many unique support chars, and only so many unique melee chars. There are only 8 slots in a party - we are never going to be able to take one of everything anyway. Classes are going to end up being underused. It will be like Mesmers in PvE (i mean how often do you see a Me spamming "LFG" and how often do you see a wammo spamming "tank LFG"), or perhaps even worse because that was just with one campaign.
c) losing Storyline - the storyline for Prophecies was reasonably good. Factions was a bit boring, but allright. Nightfall looks good. But how many games can you have where some dude(Lich,Shiro,Varesh) is trying to unleash a terrible evil on the world before people get bored?
d) Skimping on low-level PvE
Tutorial areas -
Pre-searing: about 45 quests (not including basic profession trainers), no real missions, average level when leaving: 6. However, party limit isn't eight till the Southern Shiverpeaks!!!
Shing Jea island: about 30 real quests, rest are just "Talk to ..." or "Track down " that simply exist to give you XP, two ridiculously easy missions, average level when leaving: 15, party limit is eight immediately after you leave.
Istan: about 75 real quests, three missions, average level when leaving: 15, party limit is eight immediately after you leave. I will reserve judgement till I see how long the rest of the game is.
Ascension -
Prophecies: after 17/25 missions, 5 different environments (Ascalon/Shiverpeaks/Kryta/Maguuma/Desert), a huge amount of gameplay (days and days if not months).
Factions: after 3/13 missions, 2 different environments, hardly any gameplay (couple of hours?), some people are still not even lvl 20.
In Prophecies your level gradually increases, most people are not level 20 till ascension.That's seventeen solid missions. In Factions you ascend and most likely will already be lvl 20 in the fourth mission, and the first two hardly were a challenge!
I'm not saying that Factions was "bad" - enough discussion about that - there is a very clear reason for this. To entertain experienced players coming in from other campaigns, there must be sufficient level 20 content. Factions is 90% lvl 20 stuff. Prophecies is only 50% lvl 20 stuff (people coming in by ship enter at Lion's Arch!). The amount of lvl 20 gameplay in Factions was increased compared to Prophecies, at the cost of the lower level gameplay and the length of the overall game.
Of course there has to be enough high-level stuff. But if they do this by skimping on the under lvl 20 content, what's the point? The whole learning experience is gone. For example in Factions people have such trouble with Vizunah Square, Nahpui Quarter, etc. This is because completely new players, log in, make a new character (omfg how cool an assassin now i can tele out of healing range and tank!), and with a few quests and missions are lvl 20 and thrust into the "real world" without any real experience. Of course the experienced players get frustrated, but they dont realise the reason it's like that is because ANet has to cater to them.
e) Splitting their player base - most people will be playing the new campaign, obviously. What will end up happening is 50% of the GW player base is playing nighfall, 25% is playing factions, and 25% is playing prophecies. For now its still playable. But with 5 campaigns, each with dozens of towns, to choose from, how many people will you have in the remote places? Even when there was only Prophecies, places like Ice Tooth Cave and Maguuma Stade were empty. With so many campaigns, it may eventually be hard to find a PUG even in mission locations. The Ring of Fire islands missions are mostly empty even now. Yes, new people will join, but the bulk will still be from previous campaigns.
I can think of only one solution to all this - instead of making all campaigns linked, make different "sets" of campaigns. What I mean is Prophecies, Factions, Nightfall are all linked to each other, but the next campaign will not be - it will be like another Prophecies, with a high amount of low level content. It will have its own completely different storyline. The core proffessions would be the same, of course. Then the next two campaigns will be linked to that, making another set of three campaigns just like Prophecies, Factions, and Nightfall. Then another "set" of three is produced and so on. This would eliminate most of the above problems as each campaign is only linked to two others, not 7 or 8. This could be organized by saying the storyline of each "set" takes place on a different world, not just a different continent, so they cannot be linked. Of couse the idea of three campaigns is just a random number, it could be 2, or 4. I don't think more than 5 would work though. That was ANet could still be making like 20 campaigns without the above ill effects.
I've tried explaining this to some people, their response is "ZOMG ALL CAMPAINS HAFF TO BE LINKED SO I CAN HAFF A CHAR WITH 1337 SK1LLS!!1!" Don't be like that, please.

Well if anyone actually reads everything in this monster post, congratulations! What are your opinions/ideas? Is there some magic solution?