Thanks for the comments everyone, please keep them coming!
It seems that a fair number of people feel like my campaign is a railroad, or too much of a story. In response to this, I'd like to say that my campaign descriptions are large arcs in which many adventures will occur. But I find it difficult to play in a campaign with no basic storyline present. The best campaigns I've ever played in have been well thought out and planned, with twists and turns along the way. To me, there is not much point in playing unconnected adventures one after the other and calling it a campaign.
I am prepared to change my plans, or scrap parts of the campaign if the PCs' actions lead in a completely different path, but I still think that this campaign can work more or less as it is. While I do agree that PCs can take odd, tangental paths, I also believe that once they're on a quest with a goal they generally stick to it. Sure, the journey how to get there may differ widely, but their goal won't change much.
Applying that to my campaign as an example- the first adventure will pit the PCs against a goblin clan, and hooks will be placed to make it obvious they're working as part of some larger organisation. Soon, the PCs learn of a Goblin king and his invasion. They're already enemies of the gobbos, and their homeland is at risk, why wouldn't they then decide to take down the Gobbos themselves?
Yes, I know that they may not, but eventually the Goblin invasion will be noticable, unless they travel to some other part of the world for no reason.
I see roleplaying and D&D as a combined storytelling tool, rather than a game to be won. My group and I generally think along the same lines, and that's why I'm confident a more linear storyline can work. My campaign description is just a skeleton, and the PCs decisions and actions are going to be the stuff filling it out.
As for the capture, well, I do understand that it may not go down well. It's just a scene that could work so well, and being stuck in a dungeon is such a cliched starting point of a cmapign arc that it could work. It is a difficult thing to pull off, and maybe I'll get cold feet the closer I get to it- I do have alternatives, and now with some suggestions a few more!