A difficult part of writing for me is when a single sentence–especially dialogue–contains two tones. It sounds best as a single sentence, but ending with a period, or alternative punctuation, looks wrong. As well as this, using two sentences also looks wrong.
I can’t think of a great example right now, but I know I’ve wanted punctuation that doesn’t exist before. I’ve had moments where it would have been so useful to have a “;!” and a “;?” mark.
I’ve thought the same thing many times. I love interrobang (not least of which for it’s kickass name), but the same sentiment can be expressed in other ways. Dialogue can convey the others, but if you want to recreate the way humans actually speak English, you need a mid sentence punctuation, like a comma, that can express questioning. Even in speech, we change our intonation to be higher when questioning, and we do that sometimes in the middle of a sentence. It’s just not conveyable perfectly with our current set of symbols.