Since April 2018, there were Changes to the Area 51 Process v3.0 when proposing a new site. One of them is:
Got a Community Needing Q&A? — Stack Exchange Can Help
If you have access to a healthy, enthusiastic community needing a Stack Exchange site, Area 51 is where you propose it.
With current process, the proposer needs to answer this question:
Please link to the organization or website organizing this effort:
If you do not yet have a community organized, reader and eager to build your site, please do not submit this proposal.
Now, sometimes existing SE communities have some new ideas that may not be suitable for their current sites, thus needing a new site.
Is it acceptable to only input existing SE communities, probably by linking to some related meta posts?
Note: for this specific discussion, please disregard some other possible issues like Would a NEW community based on this [current tag] be welcomed?.