One thing we are doing that is wacked is that you can play any type of D20 character from any rulebook; D20 WoTC, D20 Modern, D20 Future, D20 Star Wars, Eberron, D20 Sidewinder Recoiled, etc. Some have made up some PC classes from Star Trek.
First let me say that our group knew that this crawl would get boring after some time and to spice things up we decided to have the plance of shadow counterminus at many different areas of the dungeon.
Basically the rule is this. You can play any PC you want that is D20 compatible. We have 3 D20 modern guys, 1 Sidewinder Recoiled Sheriff (which happens to be a succubus from savage species @3rd level. She just got her wings), 1 Star Wars D20 Jedi, 1 Star Wars D20 Strom Trooper, 1 D20 Modern WWII Nazi fighter pilot, 1 D20 Future Dralazite and 1 Star Trek crew member with his phazer.
Basically we started the adventure by having everyone phase in at random times from their respective planes via the Plane of Shadow. Once they were in they had to aclimate to the environment (taking all proficiency negatives) by grabbing whatever weapons they could to survive. No one can even understand each other due to the language barriers. Everyone beemed in with firearm's, cell phones, computers, flashlights, cigarettes, etc. We still use action pts but only the D20 modern or Eberron guys can use them.
NPC's and baddies also phase in and out at random times bringing in whatever weapons they have on them. Sometimes the monsters of the dungeon aquire weapons from the dead D20 future or modern NPCs that warp in.
For instance I threw in an encounter with 6 orcs, 1 ogre and 1 hobgoblin.
1 orc had riot gear on (shield and all) he was the front man.
4 other orcs had flak jackets on but still used thier scimatars, axes etc.
the Ogre came out with a M2HB with a tri-pod and started laying down lead.
The hobgoblin carried the bullet belts for the M2HB
Lucky for the players the M2HB jammed (fumbled bad) and they killed everybody but the last orc the tossed a Frag Grenade into the party and ran to warn the rest of the orc's in the area.
It has been fun because you get to mix all the genre's together and see what happens.
I have had to up some CR's but overall people have been loving it and it has been pretty goofy.
When people die, the dungeon spits them back into the mix, completely naked loosing all their equipment. They loose 1 CON Score pt and some XP in addition to the loss of their stuff. That really makes it hard on the D20 Modern and Future guys. Suffice to say one of the 3rd level guys is a 1st level Fast/ 1st level Strong/ 1st level wizard. He got his spell book from the room with the dead skeleton at the table (My own addition to the scrolls and potions already in the room).
That is the sum of it.