Well its good to hear he's still getting work. Having read a large number of Bionicle novel's myself, I can say he's a fantastic writer, weather its Bionicle or not.
As for Greg continuing Bionicle, I think that would be a bit much to ask of him. Keep in mind this is just what I think.
One thing that I know is common in writers and artists in general, is that a lot of people don't want to work on the same thing too long. Greg wrote for Bionicle for roughly ten years. While he did get to write the serials, which seem to be more personal side stories that weren't mandated by Lego,I could easily imagine Greg being tired of writing Bionicle after so long, and wanting to peruse other stories.