So I had a thought that was cool. I'm a huge fan of fairy tail and would love to be able to do dragon Slayer magic. So I was wondering if it was a good idea to kinda mooch off of monk and add some abilities that are like the dragon slayers. Any thoughts on this would be much appreciated thanks

I've been very tempted to make custom classes in the past - but I've never succumbed before because I think it's important to ask yourself "Is the concept I have possible to do with an existing class build? If not, is my concept better as a class archetype?".

Usually the answer to one of this is always yes - especially if you're willing to dip into multi-classing. I think it's significant that the most recent 3 classes that have been introduced - Gunsninger, Artificier, Bloodhunter - have been based around introducing new facets to the game that were not really there before: technology & having a monstrous nature ( although, I think the Bloodhunter might be a bit weak in that regard ). 

I have a Character in my current Campaign that is shaping up to be Dragonslayer - Ranger with Mountains as their favored terrain, and Dragons as their favored enemy. I think they could make a kickass slayer of dragons as-is, although taking a dip into Warlock, or Monk could be interesting  - which is the vibe I'm getting from your original question. But the concept can be built using standard class system.

Now - I won't ever tell you not to do it. If you want to make a custom class for fun, and to explore and figure out how the class system works, and to see what it takes to make a balanced class, the go for it! Figuring stuff out, and customizing the game is fun.

I just think that if you're looking to introduce this class into a game, you'll need to be cautious, as making a balanced class is hard. You might try building a custom class archetype as practice. Chris Perkins has done some good interviews on building custom content, in this vein.

If you're looking at this for your character in a game, I think you can build the concept out of existing kit, without having to go through the considerable work of trying to build a custom balanced class.

But no matter what you do - have fun with it! :)

How to make a dragon slayer class:

1. Make a copy of draconic bloodline sorcerer.
2. Change name to dragon slayer.
3. Smile at your achievement.

You could do the same with way of 4 elements monk. Or multiclass.

Thanks a bunch this is really helpful!!!!!

Basically what Vedexent said. Not only does a new character class have to occupy a unique niche that the other classes don't fill, it also has to be a big enough niche to have multiple subclasses. And even if you manage to find something like that, you still have a lot of extra legwork to do: which magic items can this new class use? What are its multiclassing requirements? If someone multiclasses into this from any other the other 12 classes, is it going to break something?

Creating a new subclass is way less work, and if you can make the "dragon slaying" thing work with existing rules, even better. The Hunter Ranger in particular fits the bill, with abilities like Colossus Slayer/Giant Killer, Iron Will/Multiattack Defense, and Evasion. Plus, if the Elemental Evil/Xanathar's Guide to Everything spells are available, they also get Absorb Elements.

If you bang out something, please share it.

I actually made a Ranger subclass like this. Although they are "wizards" in the show, I feel like Rangers made more sense for their casting ability since their magic is focused on slaying a particular type of monster. 

At first it was Fairy Tail inspired, but then I also tried to really make it a proper Ranger subclass that fits in with their abilities and training more. 

You can check it out here if you like, maybe it will fit what you want: https://www.dndbeyond.com/subclasses/139373-dragon-slayer