This is impossible because different screens are different sizes. There is no-one-size-fits-all.
The key to aspect ratios in Shotcut is choosing the correct Settings > Video Mode before you start your project. If you have a specific screen size that you want to target, set the video mode to match it. If you don’t have a specific screen size to target, then you are probably better off setting the video mode to match the native resolution of your source clips. In that case, you would let the user control the aspect ratio conversion in their player (like you mention with VLC).