Q: What do I do if I'm the key of E, and I really want to hit that low Eb or D below open E on my guitar (and I don't want to go up an octave to hit the note)?
A: your choices are: 1. tune the low string down and get used to playing the entire song that way, 2. tune your entire guitar down and get used to playing the song that way, 3. overdub the part if you are recording, 4. get another player if you are performing it live, and have one of the instruments be lower, and re-distribute the parts as needed