Tank motivation is a pretty popular topic in the wake of Blizzard's latest bright idea, so here's a quick drive-by musing for folks, so here's a drive-by discussion point.
Rohan suggests that distributing the responsibility of the tanking (and healing) roles among multiple players might lessen the pressure and therefore make players more willing to tank. He suggests that 2 tanks, 2 healers and 2 DPS might work. Chris@Game By Night's experience with public rift groups suggests otherwise, as players would rather wipe and fail than switch over to healing, even when most classes have passive healing options that play almost identically to DPS.
What if we went in the opposite direction? The current holy trinity calls for 1 tank, 1 healer, and 3-4 DPS, but it seems that far more than 60% of players only want to DPS. If you're not going to blow up the entire holy trinity/aggro concept (which no current aggro-based game can do in a patch), and you can't convince players to move over to the tank category, why not change the numbers to match how players actually behave? Perhaps 1 tank, 2 healers, and 7 DPS would reflect the actual preferences of the population.
There are problems to overcome. That said, pretty much all of these games already offer a raiding format that is about twice as large as the default group but only utilities a single tank - if anything, this would lessen the degree to which single group content fosters 2-4 times more tanks than there are slots for in raids.