There is a large imbalance in the relative value/utility between the various specialist wizard signature spells. Some are so useless they do not warrant ever casting, while others are some of the most useful spells in their respective schools. Low spell level DC based spells that have no effect unless a save is failed, should probably not be considered for inclusion as sig spells, given the saves of PCs and PvE content in the module. It is effectively handing someone a rubber chicken as a weapon.
Abjuration: Globe of Invulnerability and Greater Dispelling
Both are incredibly useful in PvP and PvE. No change needed, both are viable spells people will use. Probably one of the stronger sets of sig spells.
Conjuration: Acid Fog and Cloudkill
Both are incredibly useful in PvP and PvE. No change needed, both are viable spells people will use. Probably one of the stronger sets of sig spells across all specialists.
Divination: True Seeing and Feeblemind
True seeing incredibly useful, feeblemind less so. True seeing on CD is so valuable (especially with extended duration) having something weaker like feeblemind may be a fair tradeoff here, but it doesn't get much mileage. With nerfs to scry div spec is hurting, and could probably use more utility from its 2nd sig spell.
Illusion: Phantasmal Killer and Illusory Fall
Phantasmal killer is so-so in PvE, Illusory fall is useless and offers almost no value anywhere. These two are the standouts as weakest, being DC dependent. Illusory fall at the least should be replaced with something useful like Improved Invisibility (seems commensurate with True Seeing access from Div spec). Probably one of the weakest sets of sig spells across all specialists.
Enchantment: Good Hope and Crushing Despair
Good hope offers strong utility, Crushing despair can be useful, but is underwhelming. Perhaps something like a hold monster or mind fog would be nice instead of crushing despair, but both do offer some utility at least. A bit weaker than the others except Good Hope with its improved potency.
Evocation: Isaac's Greater Missile Storm and Bigby's Forceful Hand
IGMS is incredibly useful, Bigby's Forceful is so-so, but has value. No change needed, both are viable spells people will use.
Necromancy: Circle of Death and Undeath to Death
Undeath to Death is useful, albeit situational. Circle of Death is not that useful. Something like Predator, Withering Ray, or even Negative Energy Burst would probably offer way more value/utility.
Given the cooldown inherent to these, providing more valuable spells for some of the other specialists shouldn't present any balance considerations. It will add some utility where sig spells are effectively adding zero value currently, resulting in people actually using said features. Certain buff spells like Predator or Imp Invis being on CD, while powerful, really just result in saving a spell slot or two for something else, which Wiz could definitely use.