Sweet-expressions (t-expressions)

"Recently, using indentation as the sole grouping construct of a language has become popular".

Given that popularity doesn't say a peep about merits, it's worthless as an indicator of where to go. There are plenty of good reasons to not treat indentation as syntax. One of the more enjoyable aspects of Lisp is that that grouping is obvious, especially given an editor that knows how to count parens.