You should have no truly empty rooms. Fortunately, you seem to have already recognized that, as what you are calling 'empty rooms' are just rooms without monsters.
Rather than focus on how the room is dressed, let's focus on the substance of a room without monsters. You've already identified 2 areas of substance:
1) Information
2) Treasure
In my opinion, every room should have information in it. The information can be clues to the inhabitants and history of the dungeon - tracks, spore, claw marks on the walls, garbage piles, graffiti, sounds or odors coming from nearby rooms, or actual carvings and decoration. A lot of this will look like and act like dungeon dressing, but if you think it out and do it well, it will be both atmosphere and information. Libraries are just particularly information rich rooms, and in my opinion too information rich. Bookcases are one of the worst things to put in a dungeon because they contain vastly more content than the dungeon does! I try to avoid libraries whenever possible. But you should try to put information of some sort everywhere in your dungeon. If your game has a skill system, when you can, tying clue meaning (getting clues about clues!) to passing a skill check is a good way to keep the players engaged and reward otherwise little used skills.
The same goes for 'treasure'. It should be everywhere. There should be objects in just about every room and often they should have utility. They might just be 'reloads' of things like torches, wine, 10' poles, spell components, and ammunition or tools that the players may not have wanted to lug around, but in the right situation they are loot for the player to take advantage of. You can scatter a lot of low value stuff around the dungeon to investigate, and that makes your actual highly valuable treasure more believable rather than just having a pile of coins lying there ready to be scooped up. I have this theory that good dungeons are never cleaned out. If you do the dungeon right, the party will leave it and the dungeon will in theory remain open for business and adventures. Depending on the dungeon that might mean that a lower level party might come in and scoop the 'treasures' the higher level party missed, and fight the foes that higher level party didn't find worth their time. Or it might mean that a higher level party can penetrate areas that the lower level party had to avoid. There are all sorts of ways to hide treasure. When my current group was about 4th level, they missed one of the largest treasures I'd offered to that point because it didn't look like a treasure. It was a stack of sacks containing coffee. There were 40 bags. Each bag weighed 50 lbs. Each pound of coffee was worth a days wages. In typical D&D terms, the coffee was worth about 6000 g.p. The PC's didn't realize it. Even more to the point, they missed that the whole dungeon was a building that they could effectively steal and take title to.
Ok, so what else can you have in empty rooms?
3) "Specials": A true Gygaxian dungeon will have a lot of rooms of weirdness. A "special" is a cross between a trap and treasure. Unlike a trap, it's not hidden. Unlike a treasure, it's not pure win. It's something that you probably shouldn't play with, but a curious or daring player might. They'll probably regret it, but then again they might be able to make use of it. Although I don't approve of the overall design of many of their dungeons, the idea of the "special" is something the Lamentations of the Flame Princess people get. The number of specials out there and how you dress them is almost infinite. Like information, tying special use to the skill system is rewarding. But, some ideas:
3a) 'cursed' treasure: This is loot with a down side. It's dangerous in some manner. Green slime is an example. It's a hazard, but its also infamously weaponizable if you dare. But it could be literally cursed, or it could be contaminated (radioactive, diseased, etc.) Another simple example might be a basin containing unholy water. It's potential loot, until your paladin sticks his hand in it, in which case it is a trap. Springs of magical or cursed water, and unusual magical fungus or plants are other good examples.
3b) "slot Machines": Gygax used these more often than I do, but they are still sometimes interesting. Basically whenever you touch these, you get a random result. Sometimes the result is positive, and sometimes the result is decidedly negative.
3c) "red buttons": These are things that the players obviously should not mess with, and where obviously bad things will happen if they do - the glass case that is swarming with deadly scorpions. The demonic idol with the ominously glowing red eyes. The barrel of glowing green sludge. The big bronze gong or bell with the hammer next to it. That horrid monster that's been frozen in ice, caught in a magic mirror, or turned to stone. They practically or sometimes literally have signs with them that say, "Do not press the red button". Players will mess with them anyway. One interesting thing about red buttons is that besides being totally bypassable/optional hazards that add interest to a dungeon without necessarily adding difficulty, in the right situation, they can be turned against the bad guys, even if its something as simple as cutting the rope that drops the chandelier or casting a fire spell that causes the wall of ice to collapse.
3d) "levers": Similar to red buttons, but not actually self-inflicted damage, a lever is something that controls the dungeon environment in some fashion. Maybe the room is a guard chamber, and it contains the means to lower or raise portcullis in another passage in the dungeon. Maybe its a spillway for a dam and it allows the player to control the height of the water in a stream passage. Maybe they can unbar or unlock the otherwise difficult to pass door in an adjacent corridor. Maybe it raises or lowers an elevator room elsewhere, or engages or disengages a pit trap. Computer games use these all the time as puzzles to control access to deeper layers of the dungeon.
3e) Puzzles: Speaking of, a door that can only be easily opened if you answer a riddle or put the right colored peg in a hole or figure out how to remove the wall of thorns from in front of it is a sort of special.
4) Secret Doors: The empty room actually contains a hidden exit that allows the party access to a different area of the dungeon or provides a shortcut. Effectively, the 'room' is a branch in the dungeon that isn't immediately obvious, and therefore it isn't empty because it contains something to investigate and a choice. The secret door can be almost anything. Your "latrine" idea contains one, depending on the construction of the latrine. Most latrines lead only to bad ends, but every once in a while you could have one lead to an underground river or a sewage network that connects remote areas of the dungeon. There are tons of variations here - pits, chasms, trapdoors, airshafts, exits located high up on walls, chimneys, wells, springs, sumps, pipes, collapsed passages, walled off passages, plastered over doors, etc.
5) Traps & Hazards: I'm not sure whether you were counting these as empty or not, but in the event you've overlooked it, a room can contain some hazard and therefore isn't empty. For example, a room that consists of a narrow slippery ledge on a cliff above a chasm is not empty. The players have a problem to figure out that might consume resources. And as the room with the narrow slippery ledge above a chasm shows, a single room with even a simple idea can contain secret doors (what's that the bottom or top of the chasm? do any other passages intersect the cliff in unobvious ways? What's across the chasm?), traps (the slippery ledge), and all sorts of other things (the remains of adventures that fell into the depths below? pitons and climbing gear abandoned in the rocks? A long forgotten sack of coins left on a ledge? a flock of 'harmless' bats that will be disturbed by bright lights or loud noises?).
There are all sorts of way to go from dungeon concept to concrete rooms. You can build the concepts/rooms first, then figure out how to frame them. You can figure out the framing, list all the possible rooms that belong with that idea, and then pare off all the ones you can't fill in an interesting way. You can start by drawing an interesting feature rich map, and then brainstorm ways to fill the rooms in it, revising as needed.
A recent dungeon with a magitech/lost civilization theme has among other things the following rooms:
guard room
storage
lockers
decontamination chamber
office(s)
lounge
bathroom(s)
zoological research lab
botanical research lab
alchemical research lab
mutation chamber
dissection chamber
bestiary
breeding chamber
elevator
lay line chamber
containment field
donjon(s)
summoning chamber(s)
anomalous materials lab
divination chamber
control room
tool storage
pump room
power room
steam tunnel
air circulation room
panic room(s)
I try as hard as I can to avoid having any of the rooms be empty. If they really are empty, I should probably trim them out of the dungeon, but I don't always do as good of a job at this as I should, mostly because I'm not putting in the prep time that I really should be doing. (But, life.)
There are literally hundreds of different sorts of rooms you can have. The 1e AD&D DMG still contains one of the definitive works on dungeon dressing in its Appendices. The question isn't really whether you can come up with enough rooms, but whether you can fill the room and your dungeon really is enhanced by the room. If the room has no consequences and nothing that is worth playing with or taking from the room, it probably shouldn't exist. If it must exist because of the geography of the dungeon (you need a corridor here to link rooms or provide choices between several doors), you should still try your best to fill the room with some of the ideas I listed above.
