Most communities are not the sole community related to their topic, there are really often other communities on the same, or similar topics.
An example: there are a tonne of art related comunities, but only a small handful of them have links to other related communities.
It would be great if there was more of a culture of propping up semi-related communities. This would be particularly useful for helping more niche communities get rolling.
One thing what i miss from Reddit is that user recommending related subreddits or suggesting subreddits where the post would fit in.
I see this in the fediverse sometimes but it is much rarer.