unkempt verge
Tome of the Crucible is both thematically and mechanically close to chaos affinity but is currently set as materium/materium, so it does not work with chaos spell enhancement buildings, cannot be unlocked with chaos affinity and so on.
Change Tome of the Crucible to be materium/chaos with all that implies.
To replace it on the materium track, add a new T4 materium/materium tome.
Candidate 1:
Tome of the Fortress
Theme: great fortress against which the hordes of the enemy break
Effects: defensive interactions with siege system - wall upgrade, increase enemy siege project cost, reduce enemy healing in domain. Defensive spells or enchantments. Any economy boosts should be mainly draft.
Candidate 2:
Tome of Quicksilver
Theme: (magical) quicksilver bridges the gap between construct and life
Effects: Major transformation that gives construct type. Enchantment that buffs golems as new subtype of constructs. Combat cleanse/heal or resurrect for constructs. Economy boosts should be mainly mana.
gaunt forge