aRegarding the 'Wish' reaction... my thoughts is that it should be a calculated risk, but it should be a big risk. As in, more likely to go awry than well.
Some thoughts:
Receive correct item
Receive wrong item
Receive correct item, but wrong material
Receive wrong item, but correct material
Receive enchanted version of item (various beneficial buffs possible)
Receive cursed version of item. (is there a way to, say, make wielding an item cause a mood? Like picking up a cursed axe and going Berserk? Otherwise, syndromes/diseases might also be a possibility)
Warlock is killed
Warlock is transformed into some horrible monstrosity
Warlock is transformed into a random vermin
!FUN! occurs, like accidentally summoning a random HFS
Spawns a font of everflowing water/magma (as in, potentially floods the fortress, dunno if that would be possible from a reaction)
Chance to slay all living creatures
Absolutely nothing occurs
A random race sieges you
... there's really just a lot of things that could go wrong with it. Furthermore, another option is to have it be successful AND have something bad happen as well. I like the concept of the wish being a monkey's paw/evil genie style wish. Something where, no matter what happens, you're likely going to regret making the Wish.
The way I see it, using the Wish reaction is either something you a) do after making extensive preparations if it goes horribly wrong or b) do as an act of desperation. Furthermore, I don't think it should cost anything more than, at most, a soul/phylactery. The option should always be there... tempting you. Trying to lure you into using it. It fits into that concept of dark power that warlocks have going for them - potential for huge power, but at high risk.
Also... with regards to enchantments, Warlocks are always sieging and have those talismans on them - those could be another option. Something like the standard infantry boosts, but also things like immunity to fire (if that would even be possible) could be cool. Have them cost reagents, but the better enchantments take something hard to find/valuable.