He's probably just really old and sick and spends his time watching tv, eating dinner at 4 and going to bed a 7, but then Rey and her friends bother him and kill a senior citizen for no good reason.

It's probably going to be some b.s. about he was behind everything the whole time(kind of like how they tried to shoe horn blomfield in the last bond movie) and they try to make him be the ultimate bad guy of all the star wars and then rey and kylo defeat him and the end. They don't have the talent or ability to create new characters that are interesting so they resurrect old ones to try and make the new feel validated by comparison. Which Abrams did with Star Trek into Darkness.