I’m more in favour of @mb21’s proposal for container blocks, i.e.
:::name[arg]{#id .class key=value}
content
:::
But I can see the merit of !name[arg]{} for inline elements (rather than ::name[arg]{}) - it’s one less character and it sits well with the existing image syntax.
I’m not as bothered by consistency as I am by visual impact: I don’t want container blocks to be shouting at me 
and I’d like the arguments to be on the same line as the fence.