basically a "smart HoF" that could maybe even be offered alongside the classic HoF mode
How about replacing some existing lame fights (200hp goblins...) with different groups of higher tier monsters and adding unique, hla-using humanoid parties similar to some black pits and watcher's keep fights (and even that one in trials of the luremaster)
or how about something like:
undead blackguard tomb guards + liches.
noble frost salamanders that use GWW + something more menacing than ice trolls (ice golems? qorrashi?)
archer kit elves with smart ai shooting nasty projectiles along with some elven kensages
duergars could be composed of well-balanced parties, their clerics summoning fallen devas, dwarven defenders using hardiness, etc.
ToB-level giants (including some cleric ones)
some serious demons in ToTL underground part; maybe even some elemental princes summoned by the druids
true barbarians that rage and pummel you with power attacks and critical strikes.
burial isle populated with scores of death-wailing banshees and death ward dispelling nasties.