When it comes to familiar faces, we've already seen some pretty cool lore events in Cataclysm. The return of Ragnaros. Thrall battling the Maelstrom. Hanging out with Garona/Matthias Shaw in Twilight Highlands. And of course, the arduous, but successful, effort to bring Cenarius back to Azeroth from the Emerald Dream.
Now, I know Ysera and the Ancients were really keen on getting Cenarius back so he can help fight Ragnaros, but his return might actually end up being the catalyst that finally leads to outright open war between the Horde & Alliance.
Let's take a short trip back in history and revisit just why we had to resurrect Cenarius. Anyone remember how he died? He was killed, by none other than Grom Hellscream, Garrosh's daddy. And oh look, who's that in charge of the Horde now?
Grom holding Gorehowl, after killing Cenarius
Now, Garrosh wouldn't have any reason to fight Cenarius. At least, not at first. The fight between Grom and Cenarius is long over, and Garrosh would have no reason to revisit it, especially since Grom was victorious. That being said, we saw in Garrosh's first meeting with Baine that he's more than willing to honor old grudges and debts passed down from father to son. And while Garrosh has no beef with Cenarius right now, that would all change quickly if he felt Cenarius had impugned his honor...or that of his father.
Garrosh holding Gorehowl, after killing Cairne Bloodhoof
As for Cenarius, he might be a nice guy and the son of a Goddess and all, but I imagine even he's a little pissed at Hellscream Sr. for drinking the demon blood and turning him into worm food. And while I doubt Cenarius is petty enough to strike at the son for the sins of the father, I'm sure Cenarius would have some harsh (but justified) words for Garrosh about his dear old dad...words that Garrosh most definitely does not want to hear, let alone from one of Grom's mightiest enemies.
Now, it's a no-brainer that the return of Ragnaros is a major issue for both factions. I imagine it won't be too long before Cenarius and the rest of the druids hold a War Council with leaders of both factions, to discuss a joint attack on the Firelord.
My inner typographer wants to pluck out that extra space between "We" & "must".
Unfortunately, this can only go badly. We're all familiar with how short-tempered and arrogant Garrosh is. And we know his feeling about working with non-Horde. Even if Cenarius is benevolent enough not to bring up that whole BEING KILLED incident, how long will it be before he gets tired of Garrosh's attitude? One disgusted "like father, like son" comment and it'd all be over. Garrosh is pretty hot-headed, but the one thing that would rouse him to rage more than anything else in the world is somebody insulting Grom, whom he reveres. He'd lose his mind.
Keep in mind that Horde-Alliance relations, especially between the druids, are already quite strained. Remember that in the Shattering novel, the Night Elf/Tauren druid convocation was interrupted by murderous Twilight Cultists. Every Night Elf in attendance was slain, and died believing that the Horde/Garrosh had betrayed them. This isn't true, but do the Night Elves know this? Probably not.
Oh, and let's not forget Ashenvale itself, where Cenarius met his demise. He wasn't too keen to see the Orcs in Ashenvale years ago when they were just arriving; I'm guessing the Horde's current strong presence in the area isn't making the son of Malorne feel very neighborly.
There's really no positive resolution here. When Cenarius and Garrosh do butt heads (and they will), it will be lose-lose all around. If Cenarius refuses to fight Garrosh, Garrosh will assume he's weak and a coward, which will make teaming up to fight Ragnaros pretty awkward, if not impossible. That being said, would Cenarius even decline such a duel? He must be steaming inside about what happened with Grom, and here is his murderer's son, proudly waving around the axe that killed him, swaggering with power, taking over his ancient forests and apparently ordering the death of his beloved druids. Cenarius might be all too eager to teach this young pup some manners.
And if they battle, what then? Disaster. If Cenarius killed Garrosh, we players would rejoice, but many, many members of the Horde would be outraged. Garrosh is widely regarded as a war hero, and his death at the hands of a person so closely tied to the Night Elves would most definitely NOT go over well.
However, it would be even worse if Garrosh slew Cenarius. The Night Elves have never forgiven the Orcs for what Grom did, and if it were to happen again, I don't think any power in the world would stop Tyrande from trying to rip out Garrosh's throat. Not to mention that Varian would be ecstatic, since he's been looking for an excuse to go to war for ages now. And the cooler heads on both sides - Magni, Thrall, Cairne - are all gone.
I'm sure Cenarius' great power will be pivotal in battling Ragnaros, and that we will need his help if we hope to succeed. He is a demigod, after all. But there's so many past wrongs he could be furious about, and Garrosh is such an obstinate and antagonistic fool. It's the worst combination possible. It's like two trains inexorably heading toward each other on a terrible collision course, on tracks of bad blood and old grudges.
And there's nothing we can do but wait for the crash.