I'm not a fan of the go-go-stop (trivial battles until completely stopped by immunes) playstyle either (I often stopped playing HUv1.3 after NM most of the time for this reason). I'm also not a fan of quests that take more than about an hour (like HU's Baal run), but I suppose every once in a while those are alright.
Regarding novelty bosses, one of PureRage's v1.7 additions is an optional boss named Legion, who is immune to everything (he dodges all effects) until someone brings a special aura (given to you onEquip via a charm) close to him. Even then he can only be damaged by OpenWounds (his phys-res drops to 99%, but he regenerates HP so OWounds is definitely the way to go). His sub-name description gives players a reminder of these facts. He is a stationary boss that summons player-like clones to battle for him. While testing, I thought he was very well done and novel for HU, so hopefully you'll agree that although v1.7 and other mods have a novelty-boss like this in common, at least PureRage's was done well
Back in 2013, I played PoExile a little, but most of the features I liked in it were things I'd already changed in my modded version of HUv1.3, (I did steal their "HP/MP onStrike" concept though, as soon as PureRage gave me a hint on how that could be done). Unfortunately PoE felt like it quickly degenerated into an endless item grind, so I stopped with a couple chars around level 70. I consider PoE a very well done mod of D2, but it didn't capture my interest for long. Their Map system for generating end-game play areas is excellent.