Since we receive new dungeons and bosses over and over there are already a lot of past bosses in this game.
I would kind of like the idea to get [Skyrmish] back as a [hard/extreme mode] trial where you fight random bosses or very hard notorious monsters one after an other in a very limited amount of time. in FFXIV 1.0. version of Skyrmish you had 30 minutes to kill all bosses.
Maybe also as [hardmode/extreme] versions of their original counterparts (more damage / more HP)
In the end you may receive special skyrmish weapons.
I think it would be a nice change in contrast to normal dungeons where you fight through hordes of (trash) monsters and just in the end you have to fight against a boss.
Additionally your dps and knowledge about past bosses need to be high in order to kill these bosses in a short time limit - which should be challenging.
Skirmish in Final Fantasy XIV 1.0. :
Skirmish 1.0. Video - 1
Skirmish 1.0. Video - 2
And how about implementing even FATE Bosses maybe like Ixion and Odin to the bosses? What would you think about a boss trial time challenge?