Potential Tactical Space

If you're playing a game with lots of empty dungeon rooms, then you should consider all of those potential combat zones. And you should consider making them have tactical value in some way that maybe players or smart monsters might try to move the fight into a location better suited for them.

Narrow bridges, chokepoints, places with lots of pillars and cover, small cramped rooms, large expansive rooms, rooms with rows upon rows of shelves, etc. Furniture, to hide behind, throw, or push up against doors, and so on.

All of the [usual things that go around tactical dungeon combat][1] apply in empty rooms, it's just you have to balance how much work you're putting into prepping given that it is less likely to actually get used.

Clues

Whatever monsters are nearby or tend to wander around? Clue time! Claw marks in the walls, scorched marks on wood frames, gross-but-informative-to-the-ranger piles of dung, and so on. If it's intelligent monsters, have they left graffiti, signs, or made artistic expressions anywhere? Is everything disturbingly perfectly arranged and absent of dust? (Because, the Lich is very obsessive about keeping things "just right"?)

Again, you have to use care with how much prep time and effort you put in - players tend to skip over these kinds of clues easily, or not connect-the-dots about what they mean. Prep that does not help inform or shape play, even when it's the players being heedless, is wasted prep.

History

What was this dungeon before? Who built it? Who dwelled here before it was abandoned? Was it abandoned all at once? Was there a betrayal and a civil war between two groups? Was it a magical experiment gone wrong? A disease that wiped everyone out?

This becomes pretty interesting if you can have things like written words carved into walls or ways for the players to decode what happened, and players who like such things. This is the one players usually ignore the most, so you have to be careful about prepping it. Unless you're going to house rule a reason for players to try to puzzle this stuff out ("XP for every relevant history fact you find about this place!"), it becomes things most players don't even think about. [1]: https://bankuei.wordpress.com/2014/07/22/dungeons-part-six-monsters-hazards-and-stunting/

Potential Tactical Space

If you're playing a game with lots of empty dungeon rooms, then you should consider all of those potential combat zones. And you should consider making them have tactical value in some way that maybe players or smart monsters might try to move the fight into a location better suited for them.

Narrow bridges, chokepoints, places with lots of pillars and cover, small cramped rooms, large expansive rooms, rooms with rows upon rows of shelves, etc. Furniture, to hide behind, throw, or push up against doors, and so on.

All of the usual things that go around tactical dungeon combat apply in empty rooms, it's just you have to balance how much work you're putting into prepping given that it is less likely to actually get used.

Clues

Whatever monsters are nearby or tend to wander around? Clue time! Claw marks in the walls, scorched marks on wood frames, gross-but-informative-to-the-ranger piles of dung, and so on. If it's intelligent monsters, have they left graffiti, signs, or made artistic expressions anywhere? Is everything disturbingly perfectly arranged and absent of dust? (Because, the Lich is very obsessive about keeping things "just right"?)

Again, you have to use care with how much prep time and effort you put in - players tend to skip over these kinds of clues easily, or not connect-the-dots about what they mean. Prep that does not help inform or shape play, even when it's the players being heedless, is wasted prep.

History

What was this dungeon before? Who built it? Who dwelled here before it was abandoned? Was it abandoned all at once? Was there a betrayal and a civil war between two groups? Was it a magical experiment gone wrong? A disease that wiped everyone out?

This becomes pretty interesting if you can have things like written words carved into walls or ways for the players to decode what happened, and players who like such things. This is the one players usually ignore the most, so you have to be careful about prepping it. Unless you're going to house rule a reason for players to try to puzzle this stuff out ("XP for every relevant history fact you find about this place!"), it becomes things most players don't even think about.