I'm hoping (in my lifetime) for HL3. But I hope it's either going to have a Source engine so radically improved it will comprehensively blow us away or a complete new engine written from the ground up to do the same.Gabe already said that they will continue the saga as far as they think there is story to tell.
The Source engine is slowly evolving, which is great. But slow incriments in this way never really produce the wow factor of a new engine that represents a significant leap forward in performance & visuals.