Stuff we know.
- There is a cure for vampirism.
- The new spin-off series will focus on Klaus in New Orleans and his redemption story.
- Haley (a warewolf) will be in the new series.
- Witches will play a factor in the new series.
A good catalyst to have Klaus leave TVD and move to the new series would be to cure him. As an original Vampire/Hybrid Klaus is too powerful and angry at the MFG to just leave them without killing all of them (except Caroline). If Klaus is cured he would have good reason to be scared of all the people he has pissed off (especially Tyler atm), and he would litterally run for his life.
Curing Klaus would also explain his travel to New Orleans, a city he helped make what it is today, so that he can get help when looking for a way to become a Vampire again. Which brings into play the Witch element, as they atttempt to re-cast the original Vampire spell.
Once Klaus is no longer a Vampire he will only be a Warewolf and therefore forced to turn on every full moon, this pain, this curse could be the trigger that starts him down the road to redemption, and would explain Haley's involvment as she helps him come to grips with his new Warewolf life.
This is just my theory and is always changing, feel free to chip in
Regards Lazy Lad