Now I'm not an expert on Priests (I do have one at 100, just haven't leveled it yet) but what if we make a "Surrender to Madness" row kind of like the Druid Affinity row. You could have a
Surrender to C'thun which would modify some other aspect Shadow Priest gameplay (focused on vision and void zones?) then a
Surrender to Yogg-Saron which is like what we currently have because Yogg was all about death. The last one could be
Surrender to N'Zoth
All this stuff is just an idea but to me when you have a talent that is as cool as S2M but as difficult to balance as everyone is talking about why not just create 3 different versions an balance it around similar spells. There is also some precedence for Old Gods transforming their chosen because C'thun empowered Cho'Gall.
I also like @
Clavicle's idea for buffing the other two talents because it changes the usual brain-dead talent of
Legacy of the Void into something that has some gameplay with it by trying to stay above 60 insanity. I also liked the idea proposed before about not dying after S2M if you killed your target but that might be gameable with adds.