I think it really depends on the style you're going for, or if the song has a certain emotion to it. I would record some of the new material, replay it and figure out what emotions it evokes, and then try to come up with words and eventually lines that fit together. I've seen how some bands do it, and it's a ton of effort.
You should also figure out what kind of range the vocalist has, and what his overall style is. Some things should just not be screamed lol.
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