I'm not 100% sure but I think NirvanaHQ does auto next actions in Projects. It's a web-based system but is light years ahead of Toodledo because it's been designed specifically for GTD. Projects can be set to sequential or parallel, active/inactive.
Originally Posted by stephensaid
They have no native mobile client yet but are about to release their first iOS version (it's been in beta for at least a few months). They say that Android will follow soon after but in any case, they have a very workable mobile web version that I've tried on iOS and it's pretty good.