Looking on another thread about bedrock, I remebered factions. I suggest adding in a slime block(sponge block,iron block, basically any decent looking blast proof-ish block) that can't be broken with pickaxes, or any other tools. The only way these blocks can be broken is by placing a tnt to blow them up. I'm not sure what to make the blast resistance be(how many blasts can it take before it breaks), but the cost should be a more than obby. I doubt this would be OP since all you need to do is place the tnt, and since it auto ignites it's guaranteed to blow up. Also this would give more reasons to buy tnt other than for camping, or sky bombing purposes.