Premiere needs this stuff sooner rather than later? Delivering portrait social versions of existing commercials for - facebook particularly - can involve split screens where existing assets, like cars, can't survive outside of 16:9. There are also issues with custom title safe relative to the FB feed cut off for 9:16 portrait.
I've made overlay grids in PS for vertical thirds etc - but it would be great to the point of being obvious for PPro to provide some kind of ruler, grid and guides you could invoke in the program monitor. It comes into my head most days at this stage.
and Yes - I've hit the go wish request. Just wondering if there's more than me waiting for this.