People like their current social spaces for that sort of communication and without their friends all moving too, that's not going to end any time soon. So either you would have people splitting their content amongst multiple blogs or you wouldn't have many people moving across.

Instead what you can do is offer RSS feed aggregation of existing blogs which would mean that people would still use LJ and the like for posting and the public posts would appear on the fan archive site through these feeds. Allow users to add their LJ feed to their profile (may need human admin confirmation) and you essentially have a lot of the same value for less website development. It also means that the blog-like content is under the TOS of sites like LJ so you don't need to worry too much about the scope of non-fic content.

This will allow you to capture the conversations, which are an important part of fandom, without having to force the conversations to move out of their comfortable space - it reinforces the social communities like LJ instead of reinventing them on the new site.

Certainly, the discussions I've had regarding the frameworks for building this site, it's entirely doable for not a great deal of effort, imo.