Recently i've been watching through LotR and harry potter for the first time with my family and I've kinda gotten inspired to make my own kinda fantasy world in dnd.
It won't be 100% homebrew, just things like the map, lore, and npcs (if those are even homebrew) will be. Monsters, classes, species, and most other things will be regular 5e. The idea is a fully fleshed out open world, with no real goal, just being exploring. I'm planning for almost every monster to show up at least once, so if some people wanna fight, they can fight, and if they want to roleplay, they can do that.

Im not really sure how good this idea is tho. it'll take a lot of time and effort for what can be only a few sessions if my players sometimes don't show up and we have to cancel early. I might have some sort of system where players can drop out any time and i invite new people to take their place to solve that, but no clue. thoughts?

If you find players that want this, then its good.

My experience is open worlds tend to stagnate.

But depends on your players

My advice is to start small. Don't begin by thinking you can create an entire world from whole cloth. Start small. Build out an individual city or settlement first. Then as the party begin to explore you can add the next settlement, the next ruin, the next thing you've created. Creating your whole world is a project that will take time, a lot of time if you're doing it right. My current world for my players is Æthranti, prior to that I had Eternarii. Both took on the order of 6 months to flesh out the basics. Æthranti was actually easier because the gods don't exist per se within that world. Powerful beings certainly do, but not gods. The first campaign in that world - Burning Hunger was set all within a day's journey from a city called Forsceta. So, I only really had to flesh out Forsceta and the immediate area.

What you're proposing though would mean fleshing out the entire world before ever starting an adventure. If you want the world open to exploration, you're either going to need to plan it out in advance or improvise - a LOT. 

My advice would be to direct the exploration somewhat. Think about Lord of the Rings. We start in an area - Hobbiton. It's a small part of the shire but allows us to see a bit of what the shire looks like. The adventure then takes us through a load of different areas where we the reader can experience what other areas of the world look like. We can see the dwarven mines, human kingdoms, elven forests and such like. In real terms what I'm getting at here is that directing the exploration a little allows you as a world builder to create places with a bit more planning and lead time.

In short, yeah a full on exploration will be great with the right group, but don't underestimate the task at hand. Either give yourself a lot of time to plan and create before you put it in front of players or direct the exploration along a fixed path so that you have some time to build as you go. 

The other thing I'd add is that if you're going truly open world exploration, don't give out loads of treasure or magic items. Otherwise it's entirely possible you'll get characters realising they've got enough to retire and then asking why they should bother to continue to explore. Every character needs some form of motivation. Unless you're lucky enough to have a table of Star Trek TNG nerds who want to explore for the thrill of exploration, you'll need a reason for them to continue exploring.

I would recommend making a starting place, running some adventures, and as the start to push the borders of your starting area, build out. I'm actually doing that with my world right now- I have an active campaign, but they're only in one area, and I'm (slowly) fleshing out the rest of the world for them to explore.

Building a whole fantasy world is something you should do for you, because you enjoy it. Many players will not care and be just as happy running in an existing setting. And many campaigns fizzle after a few sessions. However, a setting you lovingly craft can be used for many different things, it could be a full campaign or just a one-shot or for writing novels or whatever you want. 

My Main advice would be to set up some major objectives, some mid range objectives and some minor objectives. That way if players wanted to run around the world aimlessly they can. but they also have the option of running a one shot (minor objective) a short adventure 4 to 5 sessions(mid range) or a full campaign 10 + sessions (Major objectives). You do not need to push the idea of the players following a story that you have decided, but it gives them direction to prevent the fizzle out party scenario. You donèt need to plan the whole thing out just come up with some basic ideas and allow your creativity to run wild as the players run amuck. 

This way too if there is someone getting bored with just random role play they don't feel like they are doing a play by post over a voice channel. They  have options and those can be decided through Role play. Same world, same random encounters and some structure for those who require it. 

Just some advice 

It could be good if there's a little like backbone of storyline otherwise the players may get bored of like just running around (no offence) so yeah mostly good 👍 

My 2 cents of this - write down a general idea of what you have in mind and why it should be cool. Then pitch it to the players. If they get excited - then continue. It could be disheartening if you spend 40+ hours prepping a fantasy world when your friends want to play Cyberpunk or Traveler. 

I run a homebrew open world for my campaign, which I am very strict with myself to prevent railroading - the party can go wherever they want.

What I also do is make sure that there are consequences for doing so - a world which continues whether they are there or not. That's what I've found works - commit to running a world, not a campaign. Keep track of how long passes and consider long-term repercussions of each event. If they leave now, what will happen? If they come back in 6 months, what will have changed?

If the party decides to ditch a boring sounding quest to save a village and then comes back later, don't expect the same NPC to be stood with an exclamation mark above their head - the decision to leave meant the village didn't get saved. I then try to weave in the slowness of information, and make new quests on the bones of the old - "Merchants heading to Innocentville have not been returning" - "Isn't that the village we never saved from Gnolls?".

Regarding "should I build a world", yes, you should - it's so rewarding. I do recommend starting with an idea of the world around, then make a single, well-fleshed-out location where the game begins. I found that when I started, I was making some cool sounding places. Once the players came onto the scene, their ideas forged the world, in a way which I couldn't have hoped to do myself:

• I'm from a mage school, but a more whimsical one than the strict one you've already made - can I come from somewhere with more whimsy?
• I'm from a monastic land in the distance where they worship dragons. The dragons are all dead? Ok, so they used to worship them, and carried on their traditions.
• I'm from the underdark, where the city is built into the roots of a great forest, which hangs upside down.
• I'm from this town you've made. My parents own a tavern with a fighting pit, would that fit in?
• I'm from a religious order which worships bahamut.

All these ideas have forged my world into more that I could have made alone, and as such, I recommend leaving the borders of your initial location fuzzy until they need to matter!

I agree with ThorukDuckSlayer. Build a starting area and let Them flesh out the world with their ideas and backstories. Players tend to think in ways you won't.