Sadrik said:
Power sources should be few and have a true broad niche that is not covered by another power source, imo. So spirit, can be part of the shadow power source- for instance. Creating a whole new source to fit a particular niche is a bad idea.
Well, I agree in principle; I think fewer sources with broad niches is a better approach to take than many, narrow power sources.
But specifically about spirit vis-a-vis shadow, I disagree. I think you can make a bunch of very different spirit based classes, that are quite different from shadow based classes. I would say that they are as closely related as arcane and divine are. Some of the disagreement might come from the definitions we're each using for "spirit"- I am not necessarily talking about undead.
Off the top of my head, I can see:
Spirit Controller- witch (lots of battlefield control, use masses of spirits to tie enemies down, damage them, distort the environment, etc.)
Spirit Striker- binder (gain specific powers from different "vestige" spirits)
Spirit Leader- shaman (spirit based, like in the Players Option: Spells & Magic 2.8E book)
Spirit Defender- houngan (voodoo dude; lets the spirits "ride" him and take him into battle)
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As to what power sources I think we'll see in the next few years:
Arcane
Divine
Martial
Psionic
Ki (possibly combined with psionic, otherwise much better defined and differentiated)
Primal
Shadow
I'd be interested in a few others, should they ever try them on:
Chaos
Elemental (only if well-differentiated from arcane, primal, etc)