As the original proponent of a Scala whitespace-delimited syntax (https://github.com/lihaoyi/Scalite, circa 2014), I agree with @curoli that the standard that needs to be reached is “zero required braces”.
To me, “sometimes optional braces” to me is significantly worse than “never optional braces”. Imagine if Scala code had a mix of required and non-required semi-colons, or if Python had a mix of indentation syntax and require curly braces. Either would be awful.
I don’t even think it would be that hard to get to “zero required braces”. For example, my original Scalite prototype handles this as such:
val result = m do a =>
...
do c =>
...
}
Where the do keyword is a stand-in to open a block in all scenarios, which is closed on a decrease in indentation. You can look at the linked Scalite repo above to see many examples of it working.
Hang-ups over an arbitrary specific english meaning of do aside (that anyway doesn’t really apply to programming languages, e.g. haskell or coffeescript), having a specific, unambiguous indentation-block-start delimiter has a very large number of advantages. As a bonus do is currently an almost-unused keyword, and any existing usages of do{...}while(...) can be mechanically substituted with while({...; ...})(); (it looks even prettier with indentation syntax, as Scalite shows).
do could also be in addition to :, rather than replacing it. If we’re ok with having multiple keywords open indentation-based blocks in difference scenarios, adding another one to elegantly handle the multi-line lambda scenario doesn’t seem like a big deal.