An idea that I have begun, and dropped, and come back to, and dropped, and detailed out for 3 or 5 or 10 levels [of experience], and dropped/not had a a chance to use, and so on... over the years is to do a way way back "traditional" Dungeon crawl.
Begin with the "1 HD/1st level" creatures on the "first level" of the dungeon with your 1st level characters. The 2HD/2nd level threats on the next level, and so on and so on, to the very highest HDs/levels. Clearly there has to be some overlap and thwarting expectations...not every encounter can (nor should) be manageable or expected as a victory. Sometimes the 5HD monster like to find a place to hunker down (or is trading with or just passing through) the 3rd level. "Running away" and "rushing through" in the dungeon crawl needs to be a reliable (and sometimes desirable) way to "overcome" an encounter/particular challenge. Leaving places not thoroughly explored and/or still guarded, to come back to when you've acquired more power/levels/better magic or weapons, I consider a basic tenet of the dungeon crawl.
This means, of course, that each "level" of the dungeon needs to be large enough to accommodate the engagement of the characters to increase experience level every level (or level and a half, I usually accounted for).
Things to keep in mind:
Distance. There should be, the further you descend, huge amounts of distance where there is literally nothing. Not every creature on a particular level, let alone between levels (of the dungeon), need be aware and interacting with any other level/creature.
Resources. The further from the surface you get, the more rare and sparce resources will become. Not just food or finding objects made from trees/wood. But the replacement or repair of weapons, ammo, armor,...breathable air? Potable water? Light sources? Spell components !?! Finding and/or trading for any/all can become issues.
Dead Ends! Places/pathways that become too small or dangerous for the characters to get through and make them "go around"/find different ways to continue their descent.
Internal consistency/reality. The 10' x 10' room holding 15 ogres should not exist. With another 10' x 10' chamber, 10' down the hall, somehow housing an adult red dragon. Anywhere/any level. That can't happen.
TRAPS & HAZARDS! Don't forget the traps and hazards. There don't have to be room after cavern after room INHABITED by something. In constructed areas/levels, put traps -mechanical and/or magical, purposely placed or leftover from some ancient forgotten inhabitant/constructor- should supply at least some of the adversity. There should be environmental hazards throughout (at least some "level appropriate," if not avoidable): poisonous gases, crevaces, rock falls, cliff faces/underground canyons, subterranean rivers/lakes to produce areas for abundant life, plant and fungi stuffs/a.k.a. potential food sources (and/or aquatic and vegetative threats), lava/magma chambers, etc...
Themes. You can break up some monotony of "passage, cavern, monster, treasure, passage, cavern, monster, treasure, passage..." by having certain levels "thematically" connected. Maybe a certain level is all or mostly Undead because of the vampire/necromancer/lich in residence. A layer (once far enough down and high enough level) that is basically stepping into the Plane of Earth... or Fire... or Shadow. Maybe a particularly large and successful tribe (or a few aligned tribes) of Orcs (Gnolls/Trog's/Hobgoblins...Giants? sigh yes, Drow, too, if you wish. Whatever level apropos foes you need) have taken possession of the majority of two or three levels, all to themselves...and their 'pets'...and a critter or two they are scared of/leave alone/can't dislodge/communicate with. So, like, 2 or 3 (or 5!) levels could all be about the "Stronghold of the Red Orc King."Maybe there's a level or two of political intrigue with vying factions or tribes that want supremacy, befriending/allying/freeing the enslaved clan of dwarves/gnomes/pechs/etc... Maybe an entire "level" is actually a "Treasure Vault," massive engraved doors that require unfathomable magics to open (unless you find the key held five levels -up or down!), like walking into a city of gold, filled with traps upon traps upon traps, unimaginable wealth, and a single [probably immortal] "big bad"...the entire level actually made (and filled with treasure) not to protect the treasure/keep would-be thieves/robbers out, but to lure the creature and entrap/keep it IN!
So, I've always thought that would be a fun thing to do. Very Gygax. Very "original intentions" of the game...but not, generally (or ever in my personal experience), how the game was actually played by anyone I know/knew.