Really, I don't see why they need to make a new engine at this point. It's the most flexible engine I've ever seen. All you have to do to see evidence of this is browse through the HL2 mod list on Moddb. I've seen people simply make new levels with it. Add new gameplay elements (SMOD comes to mind). I've seen people make the game look even better than it already does (FakeFactory's Cinematic Mod). I've seen people make (or at least, attempt to make) RPGs out of it. I've even seen people make RTS's out of it. And of course, there's Left 4 Dead. Fully maxed out, that game can easily compete with some of the current-gen shooters that are out now (except for Crysis obviously). So really, all that they have to do is update every now and again to suit their needs (which I believe is what they did for Left 4 Dead). So maybe the reason that HL2-EP3 is taking so bloody long (what is it now, almost 3 years?) is because they are making massive improvements to the Source engine.