I think I get where you are going, in that it would have been great if Superman in the early films had villains that could really challenge him physically. But even Doomsday in comics would have been too soon when you look at the legend that had to be built up first.
Then, after many decades of some physical challenges, definite mental challenges, and at times image challenges (especially Superman losing steam when it came to fan interest), Doomsday came along to sweeten the pot.
Now if you had a character appear as early as the 70's films, where Superman could be killed by him, it would have been before the hero worship-level of establishment had occurred. That would seem too soon.
And with Snyder's relaunch of the franchise, I'm not sure if right now is when you want an alien that is going to kill Superman. But a buildup to that story could be interesting. Maybe Doomsday already being out there in the universe, destroying mighty beings as he is drawn to Earth for some reason.
Just a thought.
maybe they can have doomsday escape from the phantom zone and gets captured by the government and tested on and they start to tease him through the film's.