With animal summons and demon summons getting multiple choices with the new UI similar to raising undead, there's a host of possibilities for new or existing tomes!
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Tome of Geomancy allowing you to pick from multiple different elementals depending on the terrain type (storm or tide in the ocean in particular)
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Reworked plant units so there's a T1 shield plant that evolves into the T3 entwined protector, plus a T3 plant skirmisher for the entwined thrall to evolve into, giving you a choice of either one in Tomes of Roots and/or Glades.
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Priests who worship dragons, with one for each type (fire priest, obsidian priest etc.), letting you pick one each time you summon them! If possible, only allowing them if you have a dragon or wyvern of that type? (bonus crazy idea: only have them summonable near dragons of their own type, not heroes/cities)
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Inferno puppies as an alternative to magma spirits in tome of pyromancy, because those ~~bad ~~ good boys deserve some way to be recruited in the early game! You might be hesitant to change how the very uniform T1 elemental summons work, but I think pyromancy in particular could use this change because the tome has 2 battle mages in it, not to mention the other T1 chaos tome, horde, having a ranged unit, leaving very little melee for early game chaos.