So there's two aberrants that tend to get pulverized before they can even come close to ranged attackers - hook horrors and umber hulks. But both are described as having innate abilities to help them move around the Underdark - horrors can climb really well, and hulks can burrow through the ground. Can such abilities be added passively to these creatures, to make them more challenging opponents?
I think it'd work similarly to how phase spiders go about their business of teleporting right on top of people and ruin their day. With a slight change in SFX (say, a [the Umber Hulk burrows under ground] emote and a hole appears below the creature, and the creature then re-appears closer to the attacker) this might(?) be possible, I guess. I'm unsure if it's outside the limits of the engine or not.
Similarly, I've seen DMs bring hook horrors up cliffs before to attack ranged opponents - can this be added passively?
It'll certainly make the business of hunting aberrants more fraught with danger!
People should avoid creatures entirely if they're planning on exploiting cliff mechanics to kill them, seeing as that's a pretty severe exploit. As long as you aren't kiting or utilizing cliffs, I don't think these creatures are particularly susceptible to ranged fighters.