The problem with gameplay modes like that is that people get too confused, and if you don't have enough people who actually know what's going on, it isn't very fun. Seeing how the concept of "don't let the enemy capture your flag" is so foreign to some people on public servers, I think having hunters, hunted, shield entities, etc. would probably make their head explode.
It seems lots of clever ideas come and go, but it's always the straightforward, simple gameplay (and maps) that prevail.