Lair actions help a lot, but beyond that, what things do you find to make boss fights in their lairs memorable? Obviously, we want to avoid having bosses just be bigger bags of HP. Are mooks and minions critical? More lair actions? Suggestions on how the boss will interact with objects and the environment within its lair?
All of that can work. I was listening to a DM's roundtable on boss monsters, and I found the suggestions all revolved around treating every boss monster as this unified "boss" archetype that should look a certain way.
Personally, I think that approach is the enemy of great design. Instead, I want to know what makes this villain tick, what the core of this conflict is, what is the defining feature of the scene.
For example, I ran a "quasit farmwrecker" as what you could call a mini-boss. I wanted that to feel like an exorcism where things start so slow you almost don't realize you're in combat, progressively getting more frantic and horrific as the scene progresses. I wanted the fight to be more about figuring out why this haunting was happening than trading punches. This was a
haunting boss. This wasn't a slug-fest with an orc warlord or a tactical engagement with a red dragon. This had to be spooky and require some clever thought.
When I was conceiving of the idea, I listened to Dael Kingsmill's ghost video, and designed a template based on her loose concepts about levels of manifestation & merged it with my ideas of Haunting Actions as a special type of Lair Action. Familiars were affected as if by
confusion, the quasit issued several curses, farm animals were possessed, divinations were scrambled, the quasit reactively gained damage immunity and turned invisible, whispers provoked the darkest impulses of some of the heroes... until they dug up the buried family "cat" (actually an imp who was the quasit's nemesis and used to bind it to haunt the farm) & captured the quasit in a bottle.
It took about 2 hours of play with many decision points – so consistent with what most folks seem to associate with a "boss fight" – but had entirely its own feel.