It's hard to say. There's definitely gems I'd like to cover, but I kept having situations where it ended up that gems just didn't work out; like I said in Tim Hunter's review, I don't want to regurgitate the RDocs. The barcode gem is like that; I'm using it for one of my real big clients, and it's super slick. but A) it's really only useful if you need to print barcodes and B) it's really, really obvious how it works and it's dead simple.
That being said, I wouldn't mind covering Og+Nitro; I don't think it's a Rails killer, but alternatives are always good.