I'm working on a new adventure for my 4th-level party. I'd like to use Kruthiks since I've always liked them and my party is at a good level to face them. I'm just going to type out my thought process here, and leave it for anyone who has ideas.
Player motivations: Without backstories, these are a little harder to figure out. One of my characters is a scholar who loves finding old books, but otherwise treasure is probably my most solid motivation for the party. Maybe they would enjoy finding a magic lair item to furnish their newly-acquired base of operations.
Setting: Kruthiks like underground areas, so a sewer or abandoned mineshaft would work. Sewers will require a large enough city and leave opportunities for waterworks to make encounters interesting. A mineshaft can be more remote and include collapsed passageways and crumbling structures.
The whole "this creature has been more aggressive recently so let's find out why" is kind of overdone, so let's reverse that. Maybe the kruthiks are actually beneficial waste-control creatures in the sewer. They're activity has decreased recently, causing waste buildup, a horrible smell, and disease in the city above. The smell alone would be a good motivation for the PCs if only they lived in the city...oh well.
Or maybe someone is taking advantage of their tunneling abilities. The heroes witness a small building collapse into a sinkhole created by the strangely geometric tunnels, finding out later that whoever is controlling them was testing their abilities, and plans to use them to collapse a wall to invade a city.
Or perhaps they're behaving in a perfectly natural way, responding to activity in their tunnels by myconids or the undead. This also might be a time to use a leveled-down version of an otyugh (down from 7 to 5, maybe elite).
I'm kind of liking the waste-control kruthik idea. What I need is a good reason why they would be less active. Maybe someone is using a part of the sewer for magic experiments, using the kruthiks as test subjects, maybe weaponizing them.
OR (and I'm just typing as the ideas come to me), maybe the experiments are being performed by the secretly-evil wizard's guild that will eventually be the main villain. There are plenty of applications there, I'd imagine.
This is already a wall of text, so I'll leave this as it is and post some more later. If any of you are inspired by something I've written here, please share!