Proposal: Proof assistants
Looking at the requirements for proof assistants, I'm a skeptical that it would be considered a "healthy" site by StackExchange. Looking at the sites that are already in Beta gives some requirements:
10 questions per day on average is a healthy beta, 5 questions or fewer per day needs some work. A healthy site generates lots of good content to make sure users keep coming back.
90% answered is a healthy beta, 80% answered needs some work. In the beta it's especially important that when new visitors ask questions they usually get a good answer.
Every site needs a solid group of core users to assist in moderating the site. We recommend: 150 users with 200+ rep (currently 346 users with 200+ rep) 10 users with 2,000+ rep (currently 34 users with 2,000+ rep) 5 users with 3,000+ rep (currently 23 users with 3,000+ rep)
2.5 answers per question is good, only 1 answer per question needs some work. On a healthy site, questions receive multiple answers and the best answer is voted to the top.
1,500 visits per day is good, 500 visits per day needs some work. A great site benefits people outside the community. Eventually, 90% of a site's traffic should come from search engines.
I don't know how much traffic the Lean Zulip has, but I don't know if Agda could average 10 per day.