Maybe add a checkbox that says "apply to all difficulties" on timing setup panel, but a mapper can always copy/paste the timing points fron 1 .osu into the others anyways, right? maybe add an option to the menu that does that automatically and allows you to select which difficulty to apply all timing points to? (and will overwrite it's current ones) coz it may mess up timing points on another difficulty considering some ppl like to use some inherited ones for hitsounds and stuff on different difficulties... i dont know maybe im overthinking this. however for red lines it might be a good idea to be able to copy all of them into other difficulties
tl;dr: Make it for red lines, not green lines as some ppl use green lines for hitsounds and they arent the same across difficulties