The generic stuff that has a broad common denominator will easily take hold on Lemmy as they would in any growing community (like shitposts, question threads, gaming, technology, news, image focused communities and so on).
The niche stuff will take a while to grow, more so as the niche subs are those less likely to move from Reddit (or already have communities like Discord that they retreat to). More specific communities will need to build a new base here unfortunately.