I wonder if the categories could be broken down further. This is just a quick brainstorm, and I don’t know how they would fit into your existing categories.
It might be possible to split architecture into sections:
- Famous constructions in France (Eiffel Tower, Versailles, etc.)
- Famous constructions in the US (Empire State Building, Golden Gate Bridge, etc.)
- Famous constructions of ancient times (Hanging Gardens of Babylon, Royal Palace of Palenque, Petra, etc.)
- etc.
Or instead of by region it could be by type of structure: “bridges”, “palaces”, and things like that.
For comics it could be split into groups that each have at least 100 items:
- Characters from X-Men (there are at least 199 just from the X-Men)
- Characters from Avengers
- Characters from DC worlds
- Characters from independent and imprint publishers (TMNT, Dreadstar, etc.)
- etc.
Mechanical machines could be split into:
- Simpler machines with only a few basic parts (anything from basic levers and pulleys to things like whammy bars and tuners on guitars, hammers inside of pianos, mousetraps, zippers, sledgehammers, looms, spinning wheels, plows, rakes, lassos, wheelbarrows, etc. — it could even be broken into sub-categories like “simple gardening/agricultural machines/tools”)
- More complex machines (this could include Rube Goldberg machines, mechanical clocks or clock parts, and other interesting things that can be found online like this, this, and this)
- Fantasy machines (search Pinterest for ideas)
Electrical machines could be split into:
- Early electrical machines (various radios, motors, etc.)
- Computers (including historical computers)
- Consumer electrical toys (Speak & Spell, Atari 2600, NES, Game Boy, etc.)
- etc.
Weapons could be broken into countries and historical periods. Even something like “European pole arms” has several distinct possibilities.
Vehicles could be split into multiple categories: civilian land vehicles, military land vehicles, aircraft, boats, motorless “vehicles” (bicycles, hang gliders, wing suits, parachutes, swimming flippers), etc.
There are probably a lot of animal categories: insects, mammals, fish, and dinosaurs and other extinct creatures.
Other ideas:
- Cartoon characters like Loony Toons or other historical cartoons
- Dungeons & Dragons monsters
- A franchise like Star Trek could be its own category just from the characters and devices (phasers, Enterprise, genesis device, atavachron, etc.)
- artists
- philosophers
- scientists
- novelists
- famous paintings (or characters from them)
Also check out Wikipedia’s list of lists for more ideas.