These trials remain the same…
Simple solution would be :
As with featured hero portals, this trials would change evey time.
Simple solution but instant spice up 
Thoughts?
I would like more information before i am able to decide to upvote or not.
For example :
Curently, you get 4, 6 and 8 emblems for finishing 3 stages of Trials of Justice ( or any other trials).
Lets say you need 3500 team power to finish the last stage of them. Current bosses are Friar Tuck, Joon and Justice.
Next time Trials of Justice come around, there could be other clerics and monks and the required teampower is higher/ lower. Lets say, there would be trials of Justice for season 2 and would feature Wilbur, Ariel and Mnessseus. You would need recommended team power of 3800 ( normaly 3500), so the amount of emblems would increase to 6,8 and 10 per stage as well. Map would also be underwater, which deals damage per turn when you run out of oxygen (like some examples in season 2).
Third time, trials of Justice would come around, it could be monks and clerics from season 3) or some other compsition of heroes from season 2…
If trials would recommend team for 4000 teampower, you would get 8, 10 and 12 emblems for finishing each stage.
Hardest could be for 4600 giving you 14,16 and 18 emblems…
Or being a weaker one (lets say 3200) would net you 3,5 and 7 emblems.
Weakest trial would be 3000 team power for 2,4,6 emblems.
Add in costumed heroes to trials and add a key or two as a reward.
Could be.
Anything to not make it less repetitive in every sense 