Regardless, I still feel like each instance really needs a !main equivalent, a default space for general submissions where posts can be cross posted to or from more specialized communities, providing exposure to a wider audience, while also letting link aggregators do what they do best by allowing users to rank what they think is relevant for the particular community, or general to the instance at large in this case.

Instead of naming this kind of community explicitly as !main, I like how !programming has been used for this instance instead, giving it a familiar title that reflects the instance that is it's namesake, while still encouraging this catch-all like community to remain in scope. For example, !meta would be unsuitable for this given it is already reserved for self reflection of the instance, much like Meta Stack Exchange.