One manual way to make this happen would be if we could “insert” posts, for lack of a better word. If I could simply have moderators insert/inject a second post, and then make it a wiki, I’d have the functionality I’m looking for.
Ideally, some type of bot would exist to do this automatically. Every time someone starts a topic, WikiBot autoposts the second post and populates it with something like “This is the Wiki Post. Anyone can edit this post with relevant information about the topic.”
Does the OP Wiki stay at the top of the page for longer topics? If so, that’s one reason to keep the first-post Wiki convention (lest users have to switch back to the first page of a topic to see a second-post Wiki.