A container doesn’t have a purpose until it’s decided what types of items should be placed inside the container.
By discussing generic container syntax in the abstract, there’s a risk that we’ll end up inventing something without an actual use case (that cannot be solved with other blocks).
I think warnings and spoilers can be considered special types of block quote. That is to say, I don’t think using the block quote syntax for these would be bending the association of the > syntax with block quotes too far.
Attributes could be added via consistent attribute syntax.
The main argument I’ve seen appears to be that we need a fenced block syntax. There’s an argument there, but I think it’s seperate from the argument that we need a generic container syntax. We can have one without the other.