Great epic campaign ideas are fantastic, but on their own, they don't make much of a game. From my own personal experience, great ideas can be a trap. My best campaigns have been almost entirely improvised, whereas my biggest campaign idea turned quickly into the worst failure ever.
Engage the players right now, not later
This is the most important thing at the start of the campaign. I had great plans for the future (not to mentioned a pretty cool history of the world), but nothing really cool to engage the players right now. It's a game. It needs to be fun right from the start, or they lose interest. And you might not even get to the end.
With orcs as the main theme, it makes sense to start with some orc attacks, which can be fine (though whether just combat works for your players depends on their taste!). Drop early hints that these are not your usual rampaging orcs. Perhaps they aim for libraries and other centers of learning (do they burn them to hurt human knowledge, or steal them for their own use?). Perhaps they use subtle, complex strategies. Perhaps they meet orcs that are surprisingly eloquent and intelligent (contrary to what they and everybody else knows about orcs), and yet wants them dead. Or maybe he has some far more cunning scheme, and leads the PCs down some seemingly helpful quest that will ultimately doom mankind.
Will the players reason with orcs? Will it blow up in their faces? Will they prove themselves to be far more brutal, unreasonable monsters than the orcs? Don't save all the good stuff until the end. The more good stuff you give them early on, the more awesome stuff will automatically develop for the rest of the campaign.
Don't plan plot. Plan encounters.
Plot is what automatically develops during the best campaigns. Planning it all out in advance is impossible, and trying will lead the players down a boring railroad. Instead, plan interesting encounters from which plot develops automatically.
Your idea to have them choose whether they should fight orc dominance or accept the now civilized orcs is great, but don't save that for the end. By the time you get to the end (if you ever get there), you'll have tons of other cool ideas. Lead with the cool ideas you have right now. Resolving those will lead to far more awesome ideas later, that you couldn't possibly have thought of up front.
The short term moral decisions
So my advice is: forget what plans you have for what fates the players hold in their hands later in the campaign, think about what decisions they will take early on (and worry later about how they impact the later parts of your campaign). Will they kill even the most civilized orcs they meet, thereby justifying the orcs' intention to wipe out humanity? (Would peace have been possible hadn't this band of adventurers so attrociously murdered so many orcs?) Or will they try to work with the orcs, thereby possible helping them with their unexpectedly subtle schemes? (Could the final doom have been prevented had they not trusted the orcs?) Whatever decision the players make, can drive the rest of the campaign.
The short term clues and puzzles
Of course moral decisions are only one of the levels on which to engage them. Puzzles, clues, gathering information, are another:
Drop lots of clues. Never be stingy with clues. Watch how they piece them together. That too can create new truths about your campaign world. Are the orcs mainly interested in centers of learning? Do they steal them or burn them? Do their seemingly random raids disrupt trade and communication, isolating human/demi-human nations from each other? What are the patterns? What are they looking for? These are pretty strategic in scope, and maybe more relevant later in the campaign, but early clues can be relevant early on, if orcs attack a library or raid a vital caravan. It's possible they're looking for something really specific, like a book on orc history or the orc psyche or something. Or some ritual or artifact that will make orcs smarter.
The short term fights
Of course it wouldn't be D&D without something to hit. What they're going to fight seems obvious: orcs. But where? And why? What's it about? What is at stake? Make them interesting. Use them to drop clues or make decisions. And it may be interesting to fight something other than orcs. Maybe they're not fighting the orcs directly, but other monsters (or people!) affected by the orcs, displaced by them, allied to them, being exterminated by them. That too can drop clues about what the orcs are up to.