I think it would be really cool if on a page for a series, there could be a link to a different series that is connected to it in some way. So for example, Mistborn Era 1 and Era 2 could be different series pages, and on the Era 1 page you'd have a "For the sequel series, see Mistborn Era 2" with a link embedded in it, and vice versa. Right now they're smushed into one page, which I think makes it feel more like they'd follow the same characters all the way through.

As another example, see the Kushiel's Legacy: Phedre's Trilogy series page, which suggests based on the title that there are other series but you would have to go searching for those somewhere else, and without using other tools it would probably be difficult to tell which series comes next.

I have no idea how complicated this would be to implement (maybe it's something users could designate to take some of the load off of you all?) but I think it would be really cool!