If we're talking Trimancer linkups, then I'd have two options.
Dirtamancer-Thinkamancer-Healomancer: Might create Golems or base defences with regeneration abilities, or create a power-boosted Dirtamancer with near-unlimited Juice. Other possible options as well, depending on how you want to play it. Either way you'd be looking at a powerful base defence force, just about impossible to break through. And you'd be sending out some seriously tough golems with your forces, units with (I think) zero upkeep that could seriously bolster your armies.
Foolamancer-Thinkamancer-Signamancer: The Foolamancer can make things appear however he wants them to. The Signamancer causes appearances to take on significant meaning. So, if the Foolamancer creates an illusion, the Signamancer makes the illusion real, for as long as the spell lasts. Foolamancer turns the city invisible? Thanks to the Signamancer, the city is literally no longer there from the enemy side's perspective, creating an unbreakable veil. Foolamancer conjures up an image of five hundred archers on the wall? Signamancer causes there to actually be five hundred archers on the wall shooting at the enemy. Serious gamebreaking potential, though again, only really as base defence, since illusions require the caster to be in the same hex, and I doubt you'd send a Trimancer linkup into the field.
Though if we're talking individual Caster tricks, the one the I think intrigues me most is a Findamancer. What exactly can they do? Maybe the Caster does the rounds of a city and "finds" more things in the vaults, butcheries, and so on. Maybe you send him out into the hexes adjoining the cities, and he "finds" gems or animals that can be harvested for food, enemy scouts, barbarian units that can be captured or turned. Maybe you want to know where the enemy forces are hiding, so you get him to look at a map and guess where he thinks they are, and he'll "find" them.
But of course that's just my opinion.