Teeworlds is a really small universe (700-1.600 players are playing on 300 servers). It may sounds great at first but for example Fortnite have 80.000.000 online players at the moment (!).
You will realise how small the community is very soon if you haven't already. Most servers are empty or half-empty. And most people just play vanilla CTF/DM or DDRace modes and don't really want to try anything new.
And what makes things worse is that teeworlds universe is split into 2-3 "domains" and it's not trivial to pick the right version for a mod.
People of 0.6 hate 0.7 version and people of 0.7 mostly don't know about 0.6 and don't care.
But read ahead as I have recommendations for you.
You never know how much time you will invest into your project in the future. It all starts with a small modifications and then your code will grow and your mod (I hope) will become more popular especially if you will invest more time and energy promoting it.
And the moment you will finally realise that 75% of teeworlds players cannot play your mod and will not install another teeworlds client just to join your game you will start rewriting it from the scratch to another codebase.
<aside> 💡 Depending on the size of the mod and amount of time you can spent daily on your project it porting it to the right codebase will take anywhere from 1 week to 2 months.
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And believe me this will be dark times because you will not do anything players will see, all bug-fixing and feature adding will be stopped. You can burn up or lose motivation to even finish this task. And you will need to learn a new framework in a process and some of your old classes will not work and at the start you will don't know why.
Even after you will finally finish porting there still will be a lot of minor bugs that were not present in the original version. That means even more time spent fixing it and postponing adding new features.
So why don't choose the right versions from the start and don't waste your time and energy later?
Seems to be obvious choice at the start, but...
<aside> 💡 You will get only 25% of players with it. But it's easier to start make small mods on CTF & DM.
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Choose it if you want to test your ideas fast and don't care much about a player count (but you should, teeworlds playerbase is extra small)