Here's a question for you. Why do so many dungeon maps not have toilets? You'd think that monsters would have to do their business too. Besides, how can they chomp you if they don't take care of their mental hygiene? Jokes aside, this small oversight points at a bigger problem that most fantasy worlds contain.
Most worlds lack something. And it's bigger than toilets.
It's coherence.
My worlds were the same. I mean, I did put toilets in my dungeon, but that just leads you to the next issue. Where do the monsters eat? How do they find mates? How can the dungeon actually make sense in the world and survive as an ecosystem? On a larger stage the same problem repeats.
If you take a look at a village you find ordinary blacksmiths somehow surviving in the shadow of a mage's tower (how can he possibly compete with a mage who can magic items into existence?), armies using medieval strategies when there are dragons (pike squares don't do well against area effects), and economic models based on horse and carriage when teleportation is everywhere (there is a reason we now no longer use horses). Zoom out another step and you've got another layer based on how immortality affects politics, what it does to a world when intelligent beings can fly, and what happens when almost any wound can be magically healed.
I can keep going. And I did. I'd put hundreds of hours into my worlds, but still they ended up colourful, busy, and thin. Like wallpaper.
It took me a long time to understand why. And the reason, once I realised it, was simple. I didn't have the bandwidth.
A believable world is staggeringly complex. What billions of people produce by living and arguing and trading and dying over thousands of years is nearly impossible for one person to simulate from scratch.
Even if you just make one change, figuring out all the ways that this will change the world is a huge undertaking. We just have to take a look at our own world to see the truth of that.
Take the smartphone. Some of the ways it changed us we were able to predict. But many were not. For example, who would have predicted that it would dissolve the barrier between work and free time? In hindsight, it's obvious. If you create a norm where everybody always has their phone with them, then that creates the expectation that they will reply to calls and emails in their free time. Few saw it coming. Let's try another: meeting times became more fluid. Before smartphones, you often agreed when and where to meet hours or even days in advance just in case you couldn't get hold of each other. That norm is gone now. And so meetings get shuffled and cancelled far more often.
Once I understood I didn't have the bandwidth, the solution was obvious. Let the real world in.
History, political science, psychology, sociology: these fields exist because people have devoted lifetimes to studying how the real world actually works. How institutions decay. How societies handle long-lived leaders. How fear shapes theology. The mechanisms are documented. The patterns are known. And they are richer, stranger, and more specific than anything our imaginations can conjure.
Now, I'm not talking about filing the serial numbers off real history and smuggling it wholesale into your fantasy world. That's boring. Besides, that's not what research gives you once you look under the cover. What you learn then is not that X leads to Y. You learn that there are mechanisms that explain why a particular thing tends to happen given a particular set of conditions. Those mechanisms are where we should focus our attention. Once you're familiar with them, it becomes possible to ask, 'What are the downstream consequences of introducing X into our world?' and follow the consequences all the way down the rabbit hole.
This adds depth. It does something else as well. It means you have to introduce far fewer changes to make the world truly original and different. That's because each change creates a cascade of further changes that you initially hadn't imagined. Each of those then gets the same treatment. In this way your initial change is more like a seed that spreads out into a branching network of roots which worm into every aspect of your world.
You have a group of people who can live forever? If we look at the effects of long-lived leaders in our world, we see this often leads to a group of advisors forming around them who end up entrenched with their own fiefs. Often, this leads to them monopolising power as well as restricting access to the leader. Additionally, the longer the leader is in place, the more the leader identifies with the system and believes a criticism of the system is a criticism of themselves. Together, these things lead to calcification, inefficiency and corruption.
In terms of what immortality does to societies, we're currently running a set of real-world experiments called the European Union that we can draw lessons from. Here we find older societies become resistant to change and often more self-interested. Young people find they have to wait longer both for 'their turn' politically and economically. This causes resentment and frustration. That, in turn, leads to more extremism rather than less.
The psychology of older people is also fascinating. Older people find it harder to make new friends. There are many reasons for this including being less open to new experiences, having less space in their social circles to accept new members, and the way that new arrivals struggle to understand what an established group is talking about because of the group's shared history.
Take all these elements to their immortal extreme and you end up with a government who is entrenched and difficult to access, a population that doesn't like change and where the young feel the political system leaves them out in the cold, and a cultural system that rejects outsiders. That's a very different picture from, say, elven societies in most fantasy stories.
Now, don't take this to mean you can't create the elven societies in fantasy worlds. That's not what I'm saying at all. Instead, if that's what you want you need to come up with good reasons why your elven society functions that way and not this way. And then you have to not shy away from working out what those changes mean for every other facet of their society.
This is how you create a deep and believable world. Even better, as you practise this muscle you'll find that it comes naturally and you can often start working out on the spot how a corner of your world would function based on the parameters you've set. You no longer have to constantly come up with new ideas for every little corner and every new adventure. Instead, new ideas simply fall out of what you've already created.
In our example above, you realise you want to explore what the young people do. Or perhaps you're interested in how the cadre around the leader have seized power and how they might be overthrown. Or you decide to explore what would happen to a city where the elderly reject change and then change is forced upon them.
Each of those could be the seed of an adventure or even a campaign. All we had to do to get there is look at the underlying mechanics of aging on politics, culture and psychology. There are so many more threads to pull on that will turn your world from a pretty picture into something that is coherent, deep and alive. Done correctly, you'll end up with interesting cultural practices, understandable motivations for your bad guys, and dungeons where even the toilets belong.
So stop trying to do everything on your own. Instead, let the real world in. Chances are you'll end up with something more creative and weirder than you could have imagined.