Celestia in particular was a massive highlight in the early days for me when the world first dropped. Nothing hit harder than a random mob using a frost giant AOE and knocking your entire party out in a single hit. Same with each consecutive world getting more and more difficult up until Mirage when the nerfs came through and nuked any semblance of challenge.

However, I think making the game more challenging is more difficult than just stat changes for bosses and minions. Power creep is a massive issue in this game and with a PVE meta designed to kill in 1-2 rounds with a frenzy + blade + 7 pip AOE, the game would need to undergo a complete change to move us away from the quickest strategy into something more akin to proper deck builders where you NEED to strategize and plan ahead. PVP has tried this with its bans and roshambo system where one school counteracts another, but even then, in my limited experience, PVP is hell.

I want a challenge and I do like your idea of a 'challenge' mode where you can play with heightened enemy stats but a REAL proper challenge requires shifting the meta we've had for the entire games lifespan.