As ive said before. My theory is something like this: erm... to be a sith lord you have to collect followers Mayby get some holocron like Palpatine. The others, i have no idea.
If there will be an apprentice, as i highly doubt, i would go for LS. She/he(hopefully she :D ) would probably turn on you, when he/she deems her(him)self ready. If DS