I've seen a few mods where people had varying degrees of success in importing HL2 content (player models, custom view models, etc). However there hasn't exactly been a flood of new content except in terms of maps, because the authoring tools provide only a severely limited ability to introduce new content to the game (oh shi- static props!!). This is understandable, since L4D2 is one of Valve's cashcows, so they're not going to release tools or source code that will allow the community to make new, interesting stuff for themselves (easily). Otherwise who would give them money to release new stuff?

So, why has no one gone to the drawing board with the HL2 source SDK to create a L4D2-like experience, but with headcrab zombies and pulse rifles? If its already being done, when is it expected?

I don't intend to make the idea of a HL2 L4D mod sound easy, but at this rate its the only alternative to experiencing lots of frustration in L4D2.