Fool's Gold as the illusion capstone is something I know you're looking at replacing, the documentation suggests as such. Which also means there must have been plenty of discussion around it already and I have no idea if something is planned for 0.9 or not, but in any case, I figure it's worth talking about because it's really a huge drag against my desire to start spamming illusionist because when comparing it to some of the vast improvements in the spellbook in other schools, it just doesn't feel like it stacks up. Which again, I know you're already working on, but I figure anything talked about as a capstone can be scaled down, used elsewhere etc, so even if the ship has already sailed, maybe something comes of it.
Makes units within your heroes spell effect radius appear as a random selection of one random other player's units for a short period, +duration per level. The illusion dissipates when taking hostile action.
Pros: Mindgame-y in a similar way to Dopplegangers, thematically appropriate, scales with player creativity more than any mechanical factor.
Cons: Seems like it would be very difficult to implement, ends up effectively dead against wary opponents, could also be a mechanically abusive way to reset targeting and mess with path-finding mid fight.
Switch the location of your hero with any active Doppelganger. Higher levels reduce the mana cost.
Pros: Almost no intrinsic power without player skill/creativity. Thematically appropriate, if slightly dodgy (illusion leading into warping reality is a common interpretation, but this does mess with the hard boundary you've moved things around to preserve.) High value relocation spell if used appropriately.
Cons: Scaling is split between this spell and Doppelgangers, dependent spells aren't inherently fun as they require two rounds of investment to pay off. Abuse potential exists in ultra-high level illusionists, who can effectively continuously spew out copies and use them to never get caught out.
Spawns a group of illusory assassins at the location of all enemy heroes. (Alternately, near your hero if that's too abusive) Higher levels increase xp and numbers.
Pros: Straightforward, easier to balance, readily countered through garrisoning and other assassination protection.
Highly effective but narrow offensive option for illusionists. Can even add conditions to balance, ie +assassination against enemies afflicted by psych afflictions.
Cons: People tend not to like assassination as a mechanic. Special delivery assassins, even ones that die easily seem like a rage magnet.
Afflicts enemies in the area of effect with visions of Armageddon and paradise. Higher levels increase duration (and maybe unit levels affected).
(Affliction: Rapture: Causes enemy units to aimlessly wander (as with burn), not respond to orders (as frenzy), lose all defensive statistics (similar to chaos) and unable to attack or cast spells. Might want this one to dissipate when struck for obvious reasons)
Pros: Thematically climactic, builds on awe and terror to induce something incredibly potent but inherently defensive. Feels like the kind of nuclear button an illusionist would have.
Cons: Inordinately debilitating against leaders as opposed to warriors and mages. Could effectively render high level illusionists immune to assault by anything other than heroes and titans. Would feel wildly variable in power based on race you're up against. (Dark Dwarves laugh at your parlor tricks).
And that's all I have at the moment. Even if this isn't useful at all, it was at least fun sitting down and thinking about this stuff for a while. I really love the mod and I'm looking forward to 0.9 immensely.
Hope this is at least a fun read.