Personally I would like to see RO5 die off, not because I dislike Castle, but I think it has less marketable features than RO4. Sure, there's stuff you can't do on RO4 you can on RO5, but at least it looks nicer and there'd be one OS on all machines.
RISC OS Internet Suite: Oregano 3, Messenger Pro, Grapevine (or a port of GAIM?), Video Chat Client
RISC OS Media Suite: Updated Replay (or a port MPlayer?), MusicMan, Cineworks, PhotoMan (a new image library manager)
What ever happened to Fresco? The STB version is still going strong.