I agree with cavac's reasoning for preserving continuity with PM, and I thought about this a lot many years ago while contemplating a rewrite of PM. My plan was to copy all (or most) visible content to a new site, but not to copy the node base.

So, for example, there'd be dedicated (separate tables) for posts and users. For the posts, we'd copy the doctext from PM, plus some metadata like author ID's and node ID's for continuity.

(I was also thinking about using the ltree feature of PostgreSQL, which would allow loading a thread with just one query.)