(Caveat this is of course a subjective almost aesthetic argument and maybe no one's really wrong (except this is just horrible . . . 0.25*:) )

It feels as if you've (almost) reimplemented python's terrible indentation-is-block structure but kept the requirement that you have to have a non-whitespace terminator token present. With this scheme it seems the worst of both worlds in that you both have to watch indentation levels like a hawk (maybe not a hawk because it's not whitespace semantically significant; perhaps like a slightly drunk kestrel or some other minorly impaired raptor . . .) but also still need to parse out and read for the terminator that also has to be there.

If there's going to be an explicit terminator then it should be visually interesting (i.e. exdented) so it can be picked out of the flow of the meat of the block. With this scheme I'd feel one can't skim over the contents of the block as easily because the end terminator of said block doesn't stand out so one is then stuck actually parsing the entire block text to find it / make sure the block is syntactically valid.

But again, personal aesthetics tomayto tomahto let's call the whole thing off.

And a very tangentially related aside: for command line usage I like the short_loops option for zsh which lets you elide the do/done and use a braced list instead:

for i in a b c d ; { print $i ; $i/bin/do_foo }