I think that summoning has a lot of room for some interesting effects.
What happens if a summon would create a level 0 (or lower) troop? If it would make the summon fail you could make a troop that “steps down” each time it is killed. For example, at level 20 it would have 22 magic and on death it summons a copy of itself at magic/2, so with +8 from max kingdoms it would create a (22+8)/2 = level 15 version. And when it dies, it would create a level 10, which would make a 5, which would make a 0. At 0 it would fail and stay dead, so you could see it as like a phoenix that keeps coming back weaker or a treant that buds off smaller trees… Would take some tinkering to make sure it never reaches an infinite loop where it can’t die of course.
Can the current system summon more than 1 troop in a cast? If so you could make an Ooze that casts Mitosis, which sacrifices itself to summon 2 (lesser) Oozes.
Another possibility, a “Mirror Monster” which has a a 1-shot spell called Clone which summons a Mirror Monster. Give it Impervious (spells bounce off) and maybe a version of Thorns (reflecting some skull damage) to go with the theme.
I think summons can be random, like the book for Daemons. If so, create a “Master of Imps”, uses all 6 colors and summons a random imp (spring, summer, autumn, winter, love, and eventually the purple one).
Could create a “cycle” of troops, which would be best in a new kingdom. A Blue troop with a spell that sacrifices itself, freezes all troops, and summons a Red troop. The Red troop’s spell sacs, burns troops, and summons a Green. Green poisons and summons Yellow. Yellow diseases and summons Brown. Brown stuns and summons Purple. And Purple mana drains and summons Blue. Would make for an interesting defensive setup.
Just some random ideas to show off the power of Summoning