*Note* Sorry, i know it's been a while since I've been here, aaand I'm probably going to be considered as necroing this thread, but I do like the concept of the Asp Turtle having a hidden cache of precious material and do actually wan't to continue this discussion in that direction. If there's a good way of doing this that doesn't offend any rule let me know! I'm not opposed to reposting this idea in smaller sections to focus the conversation down or to bring it better in line with the standards of this forum if need be.*
I love the asp turtle idea It could just be a medium sized volcano with a beach, and there could be hidden chests with diamonds on it.
If a way can be found to actively move the turtle then i think that if you mine into it's body and find it's heart, a collection of diamond and emerald ore blocks, and mine it, the turtle will die and sink to the bottom of the sea taking with it anything you've build upon its back and forcing you to navigate back out of it's body to reach the surface before drowning. The body should remain on the sea floor, at this point just becoming regular blocks rather than an entitiy, so that you can later go back and mine it if you can find a way to do so safely. (which i'm sure any of us here can think up at least 10 different creative ways to do.) It's head, flippers, and tail should turn into sandstone blocks with two blocks of obsidian as its eyes so that you can still kinda tell what it used to be!
The upshot to this is that there becomes a practical reason to seek out the turtle, and give players a choice, Do allow the creature to live in peace or possibly even make a home on its back and live along side it, or do i kill this inoffensive creature for the valuable treasure if its body?
Offering new input/POV to an existing thread line is not really bumping/necroing, it is actually preferable in most cases to keep related content on the same threads rather than create multiple new threads for the same basic ideas. I think you're fine as far as the rules go.