Cool idea for a project but if you want to attract developers for this I think you'll have to do better than just write out your goals for it and actually think further ahead about how you will accomplish those goals. There's a lot of things to consider when making a system like this.
The primary thing to consider is whether people will use it at all: if it's not easier than posting in a Discord channel or less time consuming to set up than it would be to plan your own scrims, it won't have any users and people will feel forced/inclined to use Discord anyway even if an alternative exists. Also important to note is that if you want to make this a service for lots of people you'll need to check if there even is any interest for this in the first place. I think your first step should be to create a poll of some sort and check if this is something people want and what kind of features they would like/find useful.
Another pitfall is the actual matching system. I think it will be very difficult to actually get the system to book scrims vs teams of equal skill or similar, because even in the same division there is quite a large skill gap between the bottom and top teams. Some teams won't mind playing vs teams far better than them but not everyone is the same in that regard. Until you can think of a solution for that I don't think a fully automatic system will work. Semi-automatic? Perhaps.
You want people to get notified about booked/cancelled scrims. Cool, but how do you want to do that? Mail? Discord? Steam? Push notifications?
How would this system work across different leagues and regions? You'd have to separate them in a proper and clear way so the system does not try and book scrims vs ozfortress teams when you play ETF2L or something silly like that.
Maybe you've thought about some of the things you've mentioned already and how you would implement it, but I feel like it's important to be very clear what it is that you want and how you want it to work. I like your concept but I don't think any developer would want to put their time into something like this (for free), just to realize that there is no demand for whatever it is he developed or that people prefer just messaging people over Steam or Discord to book scrims.
You could maybe talk to the person who made scrim.tf and pitch your idea there, while it's not really what you're looking for I think it would fit in nicely with the player availability system he made.