With the introduction of jewelry crafting & the ability to improve jewelry I think that adding monster sets to trials would give players an incentive to do veteran trials. My idea on how I believe it should work would be that just like veteran dungeons monster helms will only drop in vet & from the last boss. As for monster shoulders, since there are no undaunted keys for trials, I think the weekly coffer from each trial would be a good way of getting shoulder pieces. I still not sure if vdsa, vma & vas since they are done for the weapons.

Here are a few ideas
Foundation Stone Atronach set
1 piece-weapon dmg
2 piece-10% chance when dealing dmg to launch a boulder dealing x amount of physical dmg & knocking them back. Can happen once every x seconds

Possessed Mantikora
1 piece-stamina
2 piece-10% chance when dealing dmg to drop a poison shard at enemy's location dealing x poison dmg every second for x seconds. Enemies inside the shard take x% more poison dmg from all poison dmg. Can happen once every x seconds

Ra Kotu
1 piece- stamina
2 piece-x% chance when dealing melee dmg to surround yourself with spinning blades dealing x physical dmg every second for x seconds. Can happen once every x seconds

Rakkhat
1 piece-magicka
2 piece-x% chance when dealing dot dmg to place an unstable void on an enemy, after x seconds the void explodes dealing x magic dmg to target & launching x void orbs out dealing x magic dmg. Can happen once every x seconds

As for the Assembly general not really to sure. I kind of wanted to go the Domihaus route & have it be decided based off of with stat is higher since the General has 2 stomp attacks & 2 hand attacks.

I know there's more stam based sets but I was trying to use boss mechanics for ideas.