One interesting feature could be a scraper that pulls threads from other textboards, similar to Fuuka (warosu.org) or FoolFuuka.
Archivers serve a bit more of an obvious need on 4chan -- threads usually expire within a week, interesting comments can be removed by janitors and moderators, etc. But they can still fit into the general scope of multichan, which is to build a large database of textboard threads which can then be filtered by origin site or board (tags / topics).
simply duplicating these boards is nothing special:
.... BUT archiving them all and pooling them together WOULD be a fairly novel tool in itself, let alone as an extension of multichan.