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Mathematics & Physics Community
In my opinion, mathematics and physics are interconnected; I think almost all theoretical physicists are mathematicians and many mathematicians are interested in physics subjects.
Since Codidact is an embryonic Q&A website, I suggest that one community is devoted to both mathematics and physics to attract enough contributors to advance the community. If the community were overcrowded sometime, then it could be split into two separate communities.
There are some points should be mentioned:

This community would not be devoted to only those who are interested in both mathematics and physics. This idea is suggested to attract more contributors. Anyone having any math concern would be welcomed there.

One advantage of this idea is creating a community for physics enthusiasts; there may not be enough Codidact users to create a separate physics community.
Updated
Clarifying the presented idea, the following points are mentioned:

In my opinion, the relation between (theoretical) physics and mathematics is not similar to the relation between the other science branches and mathematics. I think it is somewhat clear that the relation between math and physics is not similar to the relation between math and ,say, chemistry; in fact the former is similar to the relation between chemistry and biology.
It can be claimed that all theoretical physicists are mathematicians. In the relationship between mathematics and physics, it is stated that
"In recent times the two disciplines have most often been taught separately, despite all the interrelations between physics and mathematics. This led some professional mathematicians who were also interested in mathematics education, such as Felix Klein, Richard Courant, Vladimir Arnold and Morris Kline, to strongly advocate teaching mathematics in a way more closely related to the physical sciences." 
It is worth noting that math subjects are very diverse, so a math community needs many active users keeping the community alive. Many theoretical physics experts have enough knowledge in various fields of mathematics, so they can contribute well and regularly to the community.

As stated in the original post, the main motivation of this idea to have an alive community. All math questions would be welcomed in this community. Honestly speaking, I am stunned why some people being eager to have a math community strongly oppose this idea and downvote it. Suppose that the math community is launched and after a while it becomes somewhat inactive. What would happen? The Codidact community team would close it down and would never pay attention to any math community idea.
2 answers
In my opinion, mathematics and physics are interconnected; I think almost all theoretical physicists are mathematicians and many mathematicians are interested in physics subjects.
This is even more true of computer science. Why not a Mathematics & Computer Science community?
 One advantage of this idea is creating a community for physics enthusiasts; there may not be enough Codidact users to create a separate physics community.
If there aren't enough physicists to maintain momentum as a separate community, there aren't enough physicists to maintain momentum as a subcommunity within a larger site.
 It is worth noting that math subjects are very diverse, so a math community needs many active users keeping the community alive. Many theoretical physics experts have enough knowledge in various fields of mathematics, so they can contribute well and regularly to the community.
If they're interested in answering nonphysics questions about integrals, group theory, etc. then they will do that on a mathematics site.
 As stated in the original post, the main motivation of this idea to have an alive community.
I don't think anyone is downvoting the idea because they're against an alive community. They're downvoting it because they don't think that the idea would help with that.
Having a clear identity is helpful in community building. Trying to fit related subjects under one umbrella confuses the identity. To take Maths & Physics as an example, a question in the overlap might have very different answers if it's intended as a mathematical question vs a physical question, and expecting new users to tag things correctly is an exercise in futility.
To finish, I think one lesson I would take from experience on another network of Q&A sites is that, while the scope of a site will always require continuing clarification and tweaking, splitting a site into two is a mess. It takes a lot of work and upsets people. Actively planning to do that before the site is even created seems crazy to me.
0 comments
Right now we don't have a good way of identifying people who would help build a new community, so let's do this: if you are interested in helping to build a math & physics site, please leave a comment describing your level of interest (casual visitor, enthusiast, expert in this subtopic within the site's scope, something else?). I'll edit them into the post later.
7 comments
Similarly you could say that physics and chemistry have very much in common, or physics and electronics. And so on. It's weird to combine two of them in one site I think. Better then to create a "natural science" site and let it cover everything, then branch off that one into several when needed. — Lundin about 1 month ago
@Lundin Thanks for your comment. Having a natural science community may be a good idea and may be worth to submit a site proposal. Please also see the "Updated". — MathPhysics about 1 month ago
" I am stunned why some people being eager to have a math community strongly oppose this idea and downvote it." Bystander here. I haven't voted on this proposal and don't plan to. But, you're not proposing a math community. You are proposing a math and physics site, which is what people are voting on. Frankly, that doesn't make any sense to me either. Separate sites do make sense. I might participate in a physics site, but wouldn't want to see heavy pure math there regularly. — Olin Lathrop about 1 month ago
I downvoted for the reasons others have mentioned. I would support either a general STEM / hard sciences site or a dedicated subject site, but restricting to math and physics only seems incredibly arbitrary. Additionally I think the physics community is fairly well supported by physics overflow already, so I'm not sure we would be benefiting anyone by including them with math. — Sigma about 1 month ago
@OlinLathrop Thanks for your comment. I also agree with you that having separate sites is a better idea. However, as I said in the post, there would probably not enough regular contributors to keep each community alive; the math subjects are so diverse that many people would be needed for a math community. By the way, if anyone were annoyed by seeing heavy pure math posts, they could easily filter tags to see only physics posts. — MathPhysics about 1 month ago