Add view of changed files to PR creation
Now that we have a changes view in the PR details list, it would be nice to have the same in the PR creation view: often when I'm submitting a PR I want to see all of the changes that will be included in that PR.
Example mockup:

This view would be updated automatically according to the target branch.
This would be huge for my team! The main reason I'm having trouble selling GitHub internally at my org is that you have to go online to preview what's in a PR. A view like this of files impacted by a PR would close a perceived/debatable (but to me critical!) feature gap with TFS/VSTS.
I know I'm a bit late, but if you still wish for this:
If you use VS code, the GitLens extension can compare files between branches without caring of the commits ahead/behind or history of them: https://stackoverflow.com/questions/42112526/how-to-compare-different-branches-in-visual-studio-code