First off my gaming group rotates DMs around and it's getting closer to my turn to run a new campaign. Now previously I've used published materials. (RotRL, Kingmaker, Attack on Myth Drannor and a few other one shots.)
But I uh opened my big mouth about an idea to have a party start at lvl 1 and have the first few levels be about simply surviving AFTER their home city gets flattened by a colossal creature. In fact lvl 1 would be "Hey it's a normal day in your profession...then earthquake...huge roar...and all hell breaks loose."
(Inspired by Attack on Titan, and hordes of kaiju movies, the new Godzilla film, Pacific Rim, Cloverfield, etc.)
What my group liked was the idea of having to collect the parts and spells to build a colossus to protect their country from more kaiju and insanely large monsters.
(most likely going to use the stats for the stone colossus from B4, though it's not going to be just get the spells and gold to do it.)
Where I need the advice is:
1. I have never built a campaign on my own from the ground up. Yes I'm setting this in Golarion but other then that, not much structure, nor experience in how to build a 'by the levels' story campaign.
2. Going by strict CR there will have to be encounters where the party might be level 1-4, but the monster may be waaaaaay higher in CR. The hope being that rather then fighting the city eating monster head on, the challenge is dealing with not getting stepped on, rescuing people, fighting 'smaller' monsters, (like Clovie's parasites for just one example) and dealing with refugees and looters passing in the wake of the monster(s)
Unfortunately I have no idea how to scale such very dangerous but not technically combat challenges to award experience.
So...tldr...How to make/plan out a campaign? Cause I got overly ambitious and stuck my foot in my mouth.
Okay, this one interests me. Here's how I would do it:
First things first, experience and levels. Don't nitpick XP. It's a handy tool, but for a campaign that isn't going to be based around the standard 4 combat encounter per day model, I find it tends to become more bookwork than needed. You have an idea when you'd like the party to level? That's when they level. The biggest thing is that leveling grants new abilities, some of which will need some time to get used to, and most of the time they feel like a reward. This makes them perfect for end of sessions after big moments. Party got through a long escape sequence from a monster and saved the civilians? Bing, level. Party went through a major quest line and got the help of a major city? Bing, level. My experience is that, depending on how often you play, once every 2 to 4 sessions tends to work.
Next up, plot/encounter layout. Start mapping out event triggers. Split the campaign into chunks. Example, we'll do Intro, Discovery, Buildup, Climax, Aftermath. Five segments, now we take each segment and break them down. Depending on how long you want each segment to last, set up and start mapping out different event triggers you want to go off. Pacing is key here. Horror campaigns and Quest campaigns can have many of the same elements, yet each feels different because the different phases last different times, and even in the specific campaign genres, you can set up different feels.
Say you want to run this with a fast and loose Intro phase, a short and scripted Discovery phase, a freeform Buildup phase, a short and tough Climax phase, and an open ended Aftermath. Start at the intro, sketch out a few encounters. A fight against looters, fleeing down a narrow road from a stampede, a kid trapped in a burning building, monsters on the loose, debris from a building collapsing. Think up a bunch of these, and after you have a bunch of ideas, start splitting them up into groups you like. Say day one, grab four or five of the ideas, scale them to the level you expect the party to be at, and set the party loose. As they go through the day, spring the events on them when the time feels right. Same for day 2, day 3, and so on.
Figure out a rough plan for each phase, and just break it down from there. You don't have to work on a whole phase at once, just break it down to smaller and smaller steps. The biggest thing is leave wiggle room for the players to do their own thing, and try to figure out at least two, if not three ways to start the next event in line, because your players are not going to think like you do. Also, think up fail states. What happens if the players don't get something done the way you planned? the wrong spell goes off, and an NPC dies, or a McGuffin gets all blowed up.
Another fun note: Environment is the one thing that is forgotten about in many games. All fights don't happen on a battlemat. Some happen in a swamp, where the players can't move as well. Some happen at night, where the players holding torches are a walking bulls eye. Sometimes the party gets the jump on a tough foe, sometimes the foe is injured. Every now and then, they come across an ally that can help out. Sometimes they come across a group of ten orcs at level one, an earthquake happens, and only five survive the building falling down on them. Dynamic environments and traps can do a lot to move the challenge slider both ways.
EDIT: Oh, and biggest tip for any new campaign writer. It's not going to be perfect, things are going to go wrong, and you will slip up. That's normal, and okay. Great campaigns aren't the ones where everything goes according to plan, they're the ones where everyone at the table has fun. Prep helps, it helps a lot, but nothing is going to get you through this more than having an idea of where you'd like the story to go and improvising when the details don't match.