I’m not exactly sure what you are wanting to do, but my personal preferance is to leave the video in whatever aspect ratio it was shot.
Any video player these days will add black bars to make the video fill the screen, this shouldn’t need to be done in the video editor.
On the opposite side of things, I also really dislike ‘zoomed’ in editing where old 4:3 videos have been zoomed in to try and make it seem 16:9. I know this has been done a lot to old tv shows recently.
That is of course just me, I know plenty of people that will zoom in on tablets and such destroying both the aspect ratio and amount of video on the screen etc