I've unsucessful tried to make several filters for implementing some GTD subcategories into Someday category, according to David Allen book:
-- "Not now but soon"
-- long-term review
-- "scheduled delay start"

But I can't do this because filter options not allow to choose "Someday" in Start Time category. I think it's the most easy improvement you can do for implementing theese useful GTD subcategories. For this reason, I suggest you add filter item: "Someday" into Start Time option in Filter options box.

Thanks.