Yo, devs!
Why not make every visit to an instance be a "new" experience every time?
Is it possible that for every time you hop in a dungeon, every boss in that dungeon has different scripts assigned to them?
In that case you wouldn't know what are you going to expect from a boss fight and you'll have to learn the tactics on the fly.
Are you talking about new/other abilities they can use? That would be the same as reworking all the boss mechanics which i would expect would take a huge amount of work to implement.
Also after a while you would know what to do even if each boss had 10 different scripts and you would be in the same spot again.
For that custom content idea here is a custom answer just for you!
Honestly, that's even worse than the guy asking for buffed dungeons with new scripts. What's the point of doing dungeons then? Bosses have tactics to show their difficulty mostly. You never know if you're about to heal up the boss or if he's gonna smash the tank like Tyrannus, you could get into POS where they all have the tactic of that second Utgarde pinnacle boss which is basically nothing, some HP reduce and a weak dot, making POS way easier than it should be. Boss tactics rely on the environment also, like Gharfrost's saronite throw in some dungeons could knock you off, leave no space to hide and what not. That's more than enough reasons to say no.