It's true. Heroes like Theif, Shaman, and Kaiju are long overdue for a legendary skill. I think it should happen because they've been around for a whole year, and the 2017 heroes are too new to have any legendary skill, imho. Anyway, here's what I had in mind based on OP's suggestions:
Master Thief: Grand Ruse - Master Thief steals an additional item with Sticky Fingers for every time Deception unsuccessfully grants Master Thief any type of shield. This can only stack up to three times. Grand Ruse resets after every wave.
Orc Shaman: Static Spheres - Orc Shaman summons one static sphere around him for every enemy in combat. Each ball of lightning bursts stuns any opponent level X and below whenever Orc Shaman or an ally is damaged by any source.
Kaiju: Mighty Presence - Kaiju can no longer be intimidated by heroes level X and below. Thus, energy drain used against Kaiju is 50% effective. If Kaiju runs out of energy at any point he gains a boost of energy for 10 seconds.
I tried coming up with fair but also badass legendary ideas. Master Thief should continue being the best thief in the world, Shaman can remain as support/DPS, and Kaiju needs to be remembered while the energy drain meta (Bone, Void, etc) is still alive.
Besides.. No one wants to see heroes go unused.
It's time to dust em off!