I'd say Mer should have top level subforum, since Mer is continuation of Meego. And in Mer subforum there should be Sailfish, Plasma Active and Nemo subsections. Sailfish is not the only system which uses Mer. Right now all these topics are placed in Meego/Harmattan subforum which makes it too messy, since it's usually flooded with Harmattan specific stuff.

Ubuntu Mobile and BB10 can have their own subforums as well.