I know it's weird, but I'd like a boxing ring, or a stadium. The person who has this place probably Thu is they're the center of public attention, and is better/can beat anyone, thus the boxing ring. Final Boss can be in the ring, and you have to have a slugfest with him

Maybe a skyscraper? Kind of like being at the top of the world, looking over one's territory to prey on the city's inhabitants down below... and the boss could be an eagle or something. I dunno. Seemed pretty cool in my head.

I wanna say some sort of compound built in and around a volcano. Leading up to a confrontation at the summit. As for what the keywords would be, I havent the slightest.

A corrupt prosecutor/judge could have a court where everything goes the way they want. That'd be an interesting concept.

Perhaps a hospital. The person would likely see others as helpless, needing their guidance to achieve any real progress. As for the last boss, clearly the hospital manager. Alternatively you could use a laboratory for a darker theme with similar effect.

The Prosecutor in P5 had a casino tho?

Well... Um... This is technically not false.

Only because of cognition

For a palace kind of like Futaba's (the owner not doing terrible things but still having distorted cognition), a labyrinth. The owner would feel unable to get close to others, emotional or in terms of ability.
(op contributes)

A cathedral; for someone with a god/messiah complex. An overly righteous and stubborn person, who yet holds a position of great authority and influence.

A house/mansion. Next to a bank, that's like the most obvious place to rob.

A I R S H I P

Ryuji's palace

A pirate ship?

Maybe it's the Persona fan in me speaking, but I - probably in a bout of late-night writing that I scarcely recall - came up with an idea of a sort of amalgam of Tartarus, the Midnight Channel, and Mementos.

I am not sure what the hell I was listening to at the time. I bet it was Persona 3's OST.

Interesting... care to share?

an underground, very destroyed hideout. the entrace would collapse as soon as someone goes through it... someone is going down with the person inside.

An ancient jungle temple, Indiana Jones style