The Ruby and Ruby on Rails forums are quite quiet; didn't turn out to be the big boom expected. I'd suggest sub-foruming them to the Other Server Side languages or Computer Programming forum (maybe even split, though IMO I'd put Ruby as a part of the Computer Programming forums).
For additions, I'm wondering about a .NET forum or sub-forum to the Computer Programming forum. C# and VB.NET are both pretty popular, and the only dedicate(ish) forum we have is the ASP.NET forum which IMO doesn't really qualify for having C# or VB specific questions within them. If that change does occur, than perhaps a merging of the ASP and ASP.NET forums with a new pre-thread title tag of .NET to indicate which are ASP and which are ASP.NET questions.
Also, I don't keep up too highly on the mobile devices myself, but what about an objective c forum or general mobile programming forum. I don't know how many members specialize in this field right now, but it may help to increase the general knowledge pool as well by drawing more knowledgeable people in too.