Thinking about the Spirit attribute and the way in which it interacts with Mysticism has led me to consider both illusions and ghosts... and how Potato Bugs are the primary practitioners of Mysticism (at least among the allied species).
It seems to me that ghosts and illusion have an important - and interrelated - place in a game/comic about war. I've been reading The Things They Carried with my students over the last few weeks, and this idea of the supernatural underpinnings of war - of the ways in which the enemy seems to defy the laws of the natural world - are a natural overlay to my setting. I consider too the classic DC Comics war stories ideas of such things as the ghost tank and the way these reflect actual battlefield stories.
Creatures of the spirit world ONLY have the Spirit stat and Mysticism, and can only use mystical attacks/actions. In contrast, they can only be impacted by/influenced by these things as well. I don't see ghosts being common or especially predatory; instead, I see them appearing as NPCs or unusual encounters that are meant to be handled in some way other than direct combat. D+D has programmed me to think of all monsters in terms of their game stats, and I'm trying to break that here. You don't fire your rifle at a ghost (and you don't go and get a special rifle or ammunition that allows you to do so...). Instead, you solve problems with a ghost through role playing and (probably) helping the ghost to solve the problem that it cannot solve on its own (no opposable thumbs you know). I do like the idea of stealing from DC comics here and including machinery (like a tank) that has been haunted by the team that once drove it... I especially like this if it's something the PCs can call upon (a flavor of divine intervention) in a real pinch... nothing like having a ethereal tank driven by immortal spirits roll in when things get hairy.
As far as illusion, this seems to be the signature of spiders (especially black widows). Whether illusion is a facet of mysticism (probably not) or a unique ability unto itself (more likely at this point), I'm not sure. I think that mysticism becomes too powerful if illusion also goes with it, but I'm tempted. I'll have to play out all of the ways in which illusion and mysticism can be used, and see if there are too many options. Right now, mysticism in play has tended to feel weaker than psionics/mentalism (whatever I end up calling it) and I need something to beef it up slightly. Whether illusion would do that I don't know yet...