I'm sure this has posted before, but it's something I wanted to drop some thoughts for.
As a Spiritist I'm having a heck of a time keeping track of what does or doesn't stack. Between Malady, Withered State, Stinger, Soul Link, Ethereal Paroxysm and Ethereal Leech I really can't figure out what in the world stacks with what and what overwrites/prevents what.
This also leads into spells like Regrowth - it has a very long tick time, and doesn't heal all that much, whereas anyone who can cast it already has Growth I mastered which is, arguably, better! And since it doesn't stack, well.. I wonder if anyone really ever uses Regrowth at all, the spell? It seems some adjusting there in terms of stacking or effect might make it worthwhile to have/cast in lieu of Growth I. If it stacked but had a lesser effect, or had a short duration but strong effect, etc.
As for Spiritist, it seems like, at least to me, school abilities should stack with spells. If not, the abilities are basically pigeonholed into "use whenever up instead of casting a spell," in the example of Soul Link. As for spells, I can see similar spells, a la Paroxysm and Leech, not stacking, but Withered State is a simple dot, and Malady is a DoT debuff, so those seem like they should stack. Same with Stinger, being just a spirit DoT, seems it should stack with most things, rather than interfere with leech/paroxysm.
Just some thoughts in fiddling around with schools and seeing what abilities and all are out there.