My idea would be to clone Meetup.com as much as possible and aggressively pitch it as a "Meetup alternative." Use similar color scheme and branding, and have a Meetup.com migration tool to import existing groups and user accounts. The changeover has to be as smooth as possible, there are many non-technical Meetup organizers. It needs to stand out from the other Meetup clones. People don't know where to go.

Really though I don't think Meetup is going to die. WeWork is selling it, there's no way the market would just let this huge captive audience of active users implode. Maybe FB wil buy it.