As bitterflux commented in my earlier post about bumping parties up to 6 people, the problem lies in the widely changing ratio of tanks in a group, going from 20% in 5-mans to around 8% in raids. It's a good point, a problem that I've been annoyed by before. By that I mean I really hate when people make good points. Er, I hate when raids don't scale properly from the rest of the game, which is most of the time.
"Unless bliz commits to requiring 3-4 full time tanks on every boss in a 25 man there really isn't any particular reason for anyone that wants to raid end game to roll a tank"
This would be hard to figure out. Maybe a Four Horsemen-style fight would do it, but how many times can we fight four bosses in different corners before we get bored? We want to bring in more tanks, guarantee them jobs, in order to encourage rolling tanks and therefore having more for smaller content.
How about part-time employment? Make this a DK fight and that a paladin fight and that a warrior fight and that a druid fight. Your tanks all get their fights, so your guild will want 4 tanks minimum. Maybe it would look more like DK-Druid fight or Warrior-DK or whatever. The idea is that if you force diversity of tanking classes, you also force a certain number of tanks. I'm hoping that your tanks aren't just switching to their DK and paladin alts.
With niches tanks could be much more varied. And fights. If a certain tank is expected for a fight, that means it doesn't have to be balanced for any of the others. That means more freedom for the designers.
Unfortunately this goes completely against "bring the player not the class" and "we want tanks to be able to tank any fight." The second isn't really a quote, but the general idea has been said before many times. What guild wants to be forced to recruit a specific class/role just to get past a certain point? Would tanks even enjoy switching specs every other fight as they alternate between useless and irreplaceable?
There are benefits to niche tanking, and downsides. Currently WoW is moving in the direction of generalized tanking, which carries costs. However, the tradeoff of the potential fun of a niche fight might be made up for by not having miserable tanks and recruitment officers.