In cs.se there has been some complaint (including by moderators) about how a lot of new Area 51 proposals seems like they might coincide/ overlap with cs.se scope, topics, tags, etc.
However it seems tricky to inform Area 51 users interested in new proposals to try out cs.se.
Idea: allow a community ad mechanism in Area 51. The idea is that this would work just like normal community ads except they are displayed on Area 51 proposals that seem to overlap existing communities. They might be approved via the standard six-vote threshold mechanism, etc. So for example cs.se users who think that a new Area 51 group overlaps with their scope, they could create a community ad for cs.se that would appear on the Area 51 proposal page to attempt to nudge users toward engaging with an existing Stack Exchange community, rather than attempting to create a new group, which in general is difficult, time consuming, and/or a very dicey proposition.
Somewhat related, Overlapping Area 51 proposals (not overlapping Stack Overflow)