This is a fun little question. I actually imagined every single OP and ED for my series, along with the visuals, just for the fun of it, and this is what I came up with:
For the opening, I always imagined something like "Goodmorning world". I don't know why I chose this one in particular, but I just like how it sounds. It's catchy enough and it gives you that shonen-y vibe. The OP would have to kind of deceive the viewer, since the story actually gets progressively darker.
Basically, it would be one of those scenarios where the OP gives you a wrong idea of the series and you get surprised once you find out how different the two things actually are (Death Parade, for example).
For the ED I imagine something like this:
The ending would be a little less deceiving and it would need to show how dark and cruel the world of the series actually is. I think it would fit perfectly for the character's struggles and goals.
