None. I've seen it done with Winnie the Pooh, Looney Tunes, Scooby Doo, and Tom & Jerry, and I do not like it one bit.
Saying that, I did enjoy Captain Planet when I was a kid, and it was all about looking after the environment. I think if that was brought back it could help teach kids about looking after the environment in a fun and entertaining way.
I think you are right - they started off in the 30s or 40s I think. And originally, I think they weren't specifically made for children. The 80s were a decade where children's TV really exploded, along with all the merchandising that went along with it.
Dungeons and Dragons and Defenders of the Earth would be awesome! Voltron would also be great, but Gargoyles tops my list, for sure! Particularly if they used the same voice actors, like Jonathan Frakes, Michael Dorn and basically the rest of the Star Trek TNG cast :D