I'm also glad that there is now a proposal process.  I made a fairly similar proposal almost exactly 5 years ago to the libraries list - https://mail.haskell.org/pipermail/libraries/2015-April/025471.html - but without the subtlety of particular backtrace representations.  Skimming the ensuing thread may still be informative.

In particular, there is one thing I would like to highlight from that old proposal.  I think it'd be good to have a standard way to represent a chain of exceptions, and build this into `catch` and `finally`.  Python and Java both have a mechanism for this, and both refer to it as a "cause" exception.  When an exception is thrown during exception handling, the exception being handled is preserved as its "cause".  I find this mechanism to be incredibly useful in Java, it has made the underlying issue much clearer in many cases, and in other cases at least provides helpful context.  I have no doubt such a mechanism would have saved me many hours of debugging exceptions in Haskell systems I've worked on in the past.

I considered commenting about that directly on the proposal, but I figure this is a better place to suggest expanding the scope of the change :) .  Totally understandable if you want to keep this proposal focused on stacktraces, but I think it'd be good to consider this as a potential future improvement.

-Michael