I was thinking something that could add an interesting level to building (or exploring if they spawn naturally somehow) would be blocks that take on the aspects of the block its touching, either barely, partially, or totally and would take a mid level tool to harvest when in basic form. Fleshing out the idea a bit, it might look different somehow depending on what block is touching it, but would still look slightly different than the "master" block if at all. I was thinking the barely adaptive blocks would look like what its taking from and thats about it, e.g. if its a liquid its taking from you'll be able to walk on top of it, if its spiked then it will have spikes (they won't do anything). Partially adaptive blocks would stay in place but have all of the special properties of the mimicked block, if its liquid then you would be able to walk/fall through it, acidic blocks would dissolve other blocks, and if the item is takes a less powerful tool (or more powerful) then you will have to match/pass the needed requirements like you would the master block. Totally adaptive blocks wouldn't mimic, but would become the same thing as the master block, so a bridge that had water hit it would totally dissolve, a bridge that has acid hit it would create a lot of damage and so on. The only three problems that I can think of right now surrounding this would be; A: How to keep it so they only take from one block (e.g. a long line of adaptive blocks mimicking dirt with a stone block on the end). B: How to get a partially adaptive block back once its taken on the properties of a liquid (because while hitting water with a pickaxe may be fun for some, it usually doesn't do much). C: How to keep people from using the totally adaptive blocks to mass farm a hard to get material. Solution for A: I think the simplest solution would be make it so that it will only take from what it finds first. So if you lay down a line of blocks and have them mimic dirt then they will be locked to that until all adjacent dirt blocks are destroyed. I can't think of a way to keep the ones that would spawn naturally from taking from the right block, but having those random could make some interesting areas (a water temple/maze thing) and would create a puzzle element if you wanted (breaking a line of dirt mimics in the middle turns the other half into lava or something). Solution for B: The best two solutions I could think of are it either requires the same tool as if it were just a partially adaptive block, or to make it so buckets would give you the block back (with stronger buckets for acid or lava if thats in there). Solution for C: the only way I can think of to fix this without making it annoying is make it so the blocks can't transform into certain blocks (have a log on really rare materials of "cannot be totally mimicked"), I don't think there should be too many blocks in this category, because the totally adaptive blocks seem like they'd be something hard to get or very rare to find. TL;DR: Blocks that could transform into other blocks would be cool.
I was thinking either some generic blob monster able to digest organic stuff and multiply/grow, or a kind of acid that could dissolve low grade materials and gain a small amount of size by doing so.
Aah, now I see what you're saying. I think that'd be an awesome idea, even if this one doesn't get in. But the monster one seems like it'd be in the enemy suggestion sub-topic, and could even be a boss if it eats weapons below a certain grade too.