I've been developing an idea for a new campaign that I want to feel more open-world than some of my previous ones, but I'm struggling a bit with the implementation.

The main premise is that there are several factions ruling over my campaign world that can be approached and defeated/allied with in any order, and when one faction is dealt with, the remaining ones scale up in difficulty: Kind of like how enemies in Zelda: Breath of the Wild rank up and become more prevalent when you clear a dungeon. On the topic of dungeons, I was thinking they had a more localized difficulty system, where non-story-central dungeons grow in strength the further they are from the 'starting area', so to speak.

While I could create six different versions of every homebrew monster stat block to prepare for any configuration, it doesn't feel like the best way to go about this. Is there an easier or better way of implementing this kind of scaling difficulty? Should the optional dungeons also scale in difficulty? I'd like to hear your thoughts about it!

I think the answer to these questions is, "It depends on how much work you want to do."

The game is designed with the notion that you'll stop using monsters as the game scales up, swapping goblins out for mind flayers, etc.

I think you could get away with some power scaling template where you can apply a consistent set of changes that make each successive set of foes more difficult (+ X Hp, + X AC, + some power), and it'd work. You'd have to experiment a little to sort out the appropriate challenges.

I think the easier path would be to take higher CR monsters and just reskin their visuals as whatever enemy you want them to look like.

The thing with open world, and with homebrew in general, is to not make stuff before you need it. So, no, don’t make 6 versions of each faction’s people. Just make the ones you need right now. Then as the game progresses, you make others if/when needed. 
Same with dungeons. Don’t populate them now, do it before the session when they’ll do the dungeon crawl. 

At the end of each session, have the players tell you where they’re going next, then prep for that choice. Don’t put major plot decisions at the beginning or middle of a session, or you need to prep for all kinds of branching narratives. 

So, in doing an open world structure like this, the real key that you'll need to be following up on is the Tier system built intot he game -- and sale according to that, if you scale much at all.

So, you would start using lower CR creatures for the first tier, then move to slightly higher ones, and so forth.

Now, if your overall structure is limited by some factor, and you can only use certain types of monsters, then what you need to do is to scale up monsters only for those tiers, and generally according tot he middle of the range for the tier.You only need to scale up the basics, as well: AC, HP, scores. The rest can remain the same, maybe toss out a few special abilities to keep things interesting here and there.

But, as Xalthu notes, you don't do this until you need to do it. At most, you prep a rough idea of what you will need to have.  This is especially important in an open world set up, because you are creating scenarios -- things that *might* happen, not things that will happen. Parties have a strong tendency to follow a very irregular path in open worlds, so prep is either something you do before hand entirely, or that you set up s that it is the least burdensome for you.

Quote from AEDorsay >>

This is especially important in an open world set up, because you are creating scenarios -- things that *might* happen, not things that will happen. Parties have a strong tendency to follow a very irregular path in open worlds, so prep is either something you do before hand entirely, or that you set up s that it is the least burdensome for you.

I'm very close to you in terms of plotting, but I actually think of it in terms of not things that might happen, but things that will happen, if the party doesn't get involved. For me its like, this is what the world's going to do, and the party is the variable that hasn't been accounted for. The party is The Mule in Aasimov terms, they will change things from what's expected.

I had an old DM who used to have his open world status mapped out on a chart. Down the side were the major players and world events. Across the top was party level. And then he filled in the boxes of what would happen to whom and when. So, if the party doesn't pick up on a thread, he had a gauge of when things would happen in the world. Then, once we players did something, he go back and make any necessary adjustments to the chart, as our actions sometimes rippled out into other rows and columns.

Quote from Former_Queen_Yvonne >>

I've been developing an idea for a new campaign that I want to feel more open-world than some of my previous ones, but I'm struggling a bit with the implementation.

The main premise is that there are several factions ruling over my campaign world that can be approached and defeated/allied with in any order, and when one faction is dealt with, the remaining ones scale up in difficulty: Kind of like how enemies in Zelda: Breath of the Wild rank up and become more prevalent when you clear a dungeon. On the topic of dungeons, I was thinking they had a more localized difficulty system, where non-story-central dungeons grow in strength the further they are from the 'starting area', so to speak.

While I could create six different versions of every homebrew monster stat block to prepare for any configuration, it doesn't feel like the best way to go about this. Is there an easier or better way of implementing this kind of scaling difficulty? Should the optional dungeons also scale in difficulty? I'd like to hear your thoughts about it!

 The scaling of factions like you're describing is always going to look weird if you think about it, because presumably there was a reason that the stronger factions didn't stomp on the weaker.

But it's still ok to do it that way. It's just going to be a bit weird at the logical level, if not the narrative one. 

If that bothers you, I suggest a more layered approach, where the major players aren't directly on stage at the beginning. Instead, the PCs interact with less-powerful sub-organizations at first, and gain the attention of the bigger players over time. This means they won't be upsetting the balance of power as quickly, but still gives them meaningful achievements.

Of course, there's also the risk that your players won't do the things you're expecting, and won't go to war or ally with any of the organizations you're expecting them to.

If you aren't doing a book for publication, you can afford to leave things in flux for a very long time -- just give narrative descriptions and don't fully stat out the monsters until you need them.

That said, while CR scaling in 5e isn't trivial, there are ways of writing up scaling monsters. A generic monster looks something like

• Strength: 14 + (CR+1)/2
• Single Attack: 1 attack for (CR+1)d8 + Strength Modifier (this will be slightly low at high CR, but it's very rare for high CR monsters to have only one attack)
• Multiattack: 2 attacks for (CR/2)d10 + Strength Modifier.