(For the record: I haven't actually played all the 6x12 combinations yet, but just about every other legal set-up has been done to death, and it's beginning to feel like Yardu isn't so much evil as just overly fond of large orange rugs.)
Over at BGG, miflhanc posted a proposal for using D&D (or other) tiles with Myth. He had tile population legends for 2x2, 2x4, 4x4, 4x8, 8x8 and 9x12 tiles. Sadly, many of the most interesting tiles in my D&D collection were 5x8, 5x10, or some other configuration he didn't cover. Adding those (and 5x16, 10x10, and other double-tile permutations) to his framework was going to be a mess, resulting in an overly complicated menu of options that threatened to slow the game to a crawl with analysis paralysis... (as if I don't have enough trouble with that already.)
So instead, I reinvented the wheel. Just two new Tile categories to cover the entire gamut of possible tilings and sizes, plus a third option for giving any Tile a small boost.
|Behold: The Wheel!|
Small Custom is sort of like the core game's 6x6, but with smaller overall rewards (7.7 less gold on average). I mandated a Lair, because frankly the game is rarely challenging if there's neither a Lair nor a Trap on the Tile. Didn't want to include a Trap because then there'd be all the trouble of trying to adapt the existing Trap diagrams to oddly-shaped and -sized Tiles. This could be a single tile, or depending on what gaming resources you have at your disposal it could be a combination of smaller tiles arranged together. Play this when the official Tiles have become too over-used, or if you want a Merchant but don't necessarily need the huge cash spike that comes from the official 6x6.
Large Custom is modeled after the 12x24, but scaled down significantly so there's a playable amount of lairs and only a Gold Hoard not a Treasure Hoard. Miniboss is optional. The payout is 1 Treasure more than the 6x12, but you'll have to go elsewhere for a Merchant. This is almost always going to be (at least) a double-tile, as few sets of gaming tiles include any single piece more than a foot in length. Use it if you want to make some sprawling maze of tile components, or if you want a big finale to a sessions that's a little more customizable than the official 12x24.
Add-On is for use when If you're looking to just adjust the shape of a layout or spike the difficulty of a Tile by a tiny bit. Attach one or more smaller (2x2, 2x3, 2x4, etc) tiles to an existing Tile, along with an accompanying Hunting Pack and 2 extra Gold when the combined Tile is Cleared. I decided I liked +2 Gold better than adjusting the Treasure count, as it better represented the very small climb in extra difficulty that a Hunting Pack represented, and seemed less likely to trigger cheesy min-maxing in the pursuit of loot. It's mainly meant for you to add-on to any Myth tile, but if you wanted to invoke it when laying out a Small or Large Custom build that's okay too.
There is one obvious downside to all this customization, which I alluded to before: Once you've opened the can of worms of using non-standard tiles and possibly laying out more than 1 or 2 tiles to make a Tile, it can slow down the game a bit. Hunting through a box of random D&D tiles for the components to make an awesome complicated room can be fun in itself, but can disrupt the flow of the game. My best advice there is to do some minor pre-game prep work, such as gathering small numbers of similarly-themed tiles into gallon ziplocks so you've got a smaller amount to think about when designing any given room configuration.
Also, in case it's not obvious: if your custom non-Myth Tile layout matches the size and shape of any of the official Myth Tiles (4x6, 6x6, 4x12, 6x12, 12x12 or 12x24) you have the option of using the same stats as the Myth Tile of the same size and shape, instead of calling it a Small Custom. For that matter, the line between Small and Large could be made blurrier without breaking anything. If you had 10x10 or 8x12 space and ran it as a Large Custom, it wouldn't break anything.