Basically this is a shout out to those experienced modders who've made a scripted campaign or story mode; something like Mav's Spira: Besaid or Rebel Ops, Eggman's Aldura series, AQT's Death Watch Bunker, etc. I'm trying to get an idea of the best way to go about this. What did you consider or focus on first; the plotline, the map design, or something else? Did you map out the entire thing in your head before you even started putting together skins/models/other assets? How much revision of plot or map design did you go through while making it? Any other miscellaneous advice you can think of?

In all my campaign-specific maps, form definitely followed function; that is to say that I designed the maps to fit the needs of the stories. Spira was a bit of a backward process, since I originally was just going to make a hero assault map that had some scripted events, but I let it blow itself way out of scope. For Rebel Ops, large chunks of the maps were built before I started scripting specifics, but there was some (necessary) back-and-forth between creating the map and writing individual, specific pieces of the story. All the overall design was in place long before the maps started, though.

None of that is what I would recommend as smart, though. As a designer, I take a very disorganized, "wild-west"-type approach to design (as do, I would imagine, 99% of the people here). This is pretty reasonable for beginners and projects with small goals, but if you're going to do something larger, I would recommend taking some time to draw up real documentation for yourself. Make a plan - write out your story, sketch some basic map designs, and then stick to your targets as closely as possible. I think that Rebel Ops turned out very well for a longer-timeframe map project, but its story suffered because of changes in direction along the way.

I'd point out that one of my better-documented-and-planned efforts was the campaign for Dark Times II, which involved significantly more maps and individual missions than any of my previous work. However, its plan came into being completely after the construction of its component maps (which was, in fact, what necessitated the planning - it would have been hard to make anything work otherwise).

To sum:
-Concept
-Design doc
-Iterative construction (build segments, script, build segments, script...)

If you want to have a solid, interesting story (and especially if you want to try any new gameplay elements), the form of your map must follow its function.

My process has basically been exactly the same as Mav's. For the Aldura maps, I set out with a basic idea of what I wanted the environment to be and what I wanted the overall outcome of the mission to be. The actual design of the map and individual mission objectives, however, were created more on the fly. In the end, the maps were designed specifically for the campaign modes. This made for a solid campaign experience, but the conquest modes suffered greatly.

With the mod I've been working on now, I'm taking a much more deliberate approach. Although I haven't actually started the process of scripting any missions, I'm putting a lot of effort into thinking about how I want to structure the campaign, and how that will affect the maps. I've decided to first create everything I need for the Instant Action modes - conquest, CTF, XL, and so on. I'm being very careful to make sure that the maps are well-suited for these modes, because they are what have the greatest influence on replay value. When the time comes to create the campaign missions, I'll take a look at each world and decide if I can use the current map, or if I need to create a separate map for the campaign mode. So in the end, I might have one Hoth map that I use for conquest/CTF, and another that I use for campaign/XL.

One last tidbit of advice, I highly recommend testing each objective or event as you go. It can become tedious, but this will ensure that you can see exactly how every single detail plays out. If you try to script a large number of complex events and test them for the first time in one playthrough, there's a good chance that you'll miss some problems or imperfections.