I was watching the first film last night with my girlfriend, and it was her first time seeing the film, and my first time in a while and I kind of saw things in a new light. I had seen the documentary and heard and saw Donner's cut and realized that both those versions had their share of issues, but I was convinced that had things gone as planned, and Donner would have stood around to complete the project, I think Superman I and II could have been a huge epic movie broken up into two parts.
It's been a while since I've seen Superman II, so my memory forgets me, but I felt like Superman I ended in a way that set up the sequel, but the transition wasn't very smooth, and the fact that the movies were released two years apart didn't help either. But I could easily see Superman I ending with Superman turning back time, only to learn that by playing God, he changed something else, which leads to Zod being freed. So now he saves one disaster by forming another, and I feel like that issue could have been played up a bit more. This could have been a great lesson for Clark to learn early in his time as Superman, that there are repercussions for his action.
Now, I think Lois learning Clark was Superman could have been pushed a bit later on int he film, so the sex scene would be negated, but the film would end with her finding out and the two of them having a Happily Ever After type ending. I know at the time, Lois knowing his secret wasn't very popular in the comics, and it wasn't as accepted as it is today because Lois was expected to be smart, but dumb enough to not see past the disguise. But I feel storywise, if the two movies connected seamlessly and were shown back to back, you could have had this epic Superman story that was 5 hours combined, and saw him do battle with both Lex Luthor and Zod.