Maybe raids in Cataclysm will have a lot more unavoidable raid damage. I don’t mean more pools of fire/slime/lava/shadow on the floor. Maybe more adds spawning or multiple mobs requiring dps to control/kite/evasion-tank/pet-tank/etc… Remember that all classes are getting larger health pools. It might not be out of the question for a rogue or shaman to have to pick up an add and off-tank for a few seconds.I like this line of thought.
Now as a general rule, Blizzard has given us hybrids a fairly equitable stake in all raid roles over this past expansion, especially compared to previous content. But as a consequence, the encounters have all been tuned to having those 15-18 DPS pretty much going full throttle blasting from pull to kill. So we may be hybrids, but are we actually doing anything a truly versatile hybrid class should? I mean, we're not popping out healing spells as part of our rotation, unless you count the pittiance from Vampiric Embrace or Judgement of Light. Our "tanking" is more often than not grabbing a mob's attention and either running or burning, something any pure DPS class can do just fine.
But what if that weren't the case with Cataclysm? What if we had boss fights that required a significant number of viable DPS/tank hybrids? Or heal/tank hybrids? I'm not just talking a DK popping icebound and deathgripping an add off the healers, I'm talking where every arms warrior, retribution paladin, and enhancement shaman switches in a shield to help offtank the onrushing swarms of adds without decimating those vital healer mana pools. Think of the design space this opens up...
Imagine a boss with a special attack that afflicts a dimensional shift into the Emerald Dream, and for everyone alive in the dream, the boss hits for less damage and can be tanked by progressively defensively weaker classes. In the dream, nightmare adds need to be tanked, controlled, and survived while the people outside burn the boss. If you need a healer in the dream, you'll need that healer to take that melee hit... lucky us, both druids and paladins come equipped with a taunt, and the defenses to survive an early hit. Later on in the fight, we'll have hunters misdirecting to the priests to keep DPS outside, and get those much-needed heals in the dream.
What strategy would be best used? The retadin goes across first to burn adds quickly while self-healing? Send a bear-form healer first to maximize early bloodlust/heroism damage on the boss? And that's just one idea from a rank amateur; just imagine what the folks who do this for a living can come up with.
See, what we know for a fact is 1) that healer mana is going to be a greater factor than ever, and 2) that health pools will be much larger relative to the amount of damage taken. We can also pretty safely assume that Blizzard wants to keep the proportions of full-time tanks and healers needed for a 25-man raid more or less constant. So based on that, it's not too out of line to see hybrids fulling an actual hybrid role instead of streamline focusing into maximizing their DPS output.
It might not come to pass like this, certainly, but it's not outside the realm of possibility either.