Half life episode 2 uses a source engine 14 where half life 2 (and CSS) uses the source engine 7. So if you want to make a new CSS with that be my guest .
CS 1.6 IMO is the best version because once CSS came out they basically completely remade EVERYTHING. If you still want that classic CS feeling then 1.6 is the way to go. I mean, don't get me wrong. CSS is awesome as far as graphics and whatnot, but there's something about 1.6 that makes me turn to that instead.
I played CS:S before playing CS 1.6 and I prefer CS:S. I think it's more like what you're used to, not the gameplay/graphics.
Graphics in CS:S are better than 1.6 but come on it's a FPS, not an adventure game lol. I still play MOH:AA online and I sometimes prefer it than CS, and the graphics are not good if you compare it to CS:S
On the subject, I don't think it is necessary, CS:S is still popular
There will be no HL3 as far as I know, the three episodes are suppose to take it's place.
As for the original question, CSS was made using an older version of the source engine. Newer games such as TF2 and HL2 EP2 use a much newer version, which has the ability for a lot more destruction and such. Who knows, maybe they will release a new CSS on the updates Source engine.
i have both CS:S and 1.6 i like CS:S better, cs 1.6 is harder, theres just somethin i like about CS:S, maybe be casue ive been play regular CS for like 6 years or more lol well since 1.2 whenever that was