Hey, I had same idea when I was trying to make something for Vanilla scene, but like ghost said, it's really bad, but teeworlds has a big lack in number of active clans/players.
But if you do think you can do this project, I could help you in some way.
For mod purpose here is my opinion :
- Use inQ mod for the math league just for the /pause feature and to avoid any consensus, or use Vanilla mod and forbid any pause during a match (that's kind of bad). Also, in inQ mod (the server are normal gametype finishing with a "*") /pause don't stop the respawn of weapons, which is kind of bad I think.
- For map : you should definitively use standart map at first, then if your league is a success, you could change about bringing something new.
- Don't forget you need to make CLEAR rules about such a league.
- I think the most appropriate match format should be 3v3 cause that could allow player without clan to make a temporair team. Also in rules it should be specified a player can't be in 2 teams.
- I think you are a bit pessimist about ESL. Just try to contact them at first and present your project. Like ghost said, it's maybe the best platform to use for such a thing, especially you wouldn't really need to make and host a proper website (which is a lots of work for a project you aren't even sure it will work).
So to conclude, I really like the idea, I can help in a lots of way (lend server / admin match / organise / etc...), so if you really wanna do this and you need help, use pm or I am on IRC 24/7 on #ddracemax on Quakenet.org.