I'm A Banana
Potassium-Rich
If the first adventure is the Village of Hommlet, the pcs then move to the Secret of Bone Hill, follow up by heading to investigate the Sinister Secret of Saltmarsh, go off to oppose the Red Hand of Doom and eventually make an Expedition to the Barrier Peaks, there might be a theme that emerges eventually, but equally likely, the 'theme' starts as "explore our new pcs' motivations in a small town", moves to "clear out a few adventure sites", then switches to "save the valley" and finally moves on to "find out what the heck is up with these weird monsters". And there's no dominant theme there, only themes for small bits of the campaign. I don't think the 'world tour' theme applies if the pcs don't actually spend a great deal of their time touring the world; if the tour, so to speak, is incidental to their adventures- they move on from town A because they get bored, from city B because of a misunderstanding with the law, stay at stronghold C for years, then move on again- then I don't think it would count. I guess the way I see it is the difference between Gulliver's Travels or Around the World in 40 Days, where the focus is the travel, and Star Wars, where there is a lot of travel, but the focus is definitely elsewhere.
See, I think what you're describing is a Theme of "Adventurers solving problems." Kind of a dungeon-of-the-week flavor, or a loose, episodic structure (with the potential to dive into longer arcs when necessary).
Which is why when you say this:
While I agree that "world tour" can be a theme, I disagree that it is in many cases, especially if the pcs drive the direction of the game. I think you need to intend for it to be the theme, or at least that the tour has to become a major focus of the game, for it to count as the campaign's theme (or a theme of the campaign, anyhow).
I'm not sure that I agree. A theme as Wyatt seems to be using the term is just a description of the kind of game you're running, the kind of characters you're playing, the kind of threats they're tackling. "A bunch of do-gooders slay some menacing monsters" is as much a theme as "Save the World."