The crop tool in Adobe Photoshop has the option to set the aspect ratio of cropping. In Premiere Pro, however, no other aspect ratios outside 16:9 are not available. When creating a new sequence, the aspect ratio is actually calculated in the UI, but you can't create a new sequence based on aspect ratio. Most people end up using PNG mattes, but this is not optimal when using non-16:9 open gate material etc.
You could have an aspect ratio dropdown next to frame size fields, with the usual aspect ratios included, along with "Custom".
When you then manually enter a number to either of the frame size fields, the value for other field is calculated automatically. Yes, you can use a calculator, Google or memorize the frame sizes, but a dropdown would make it much more painless.
What you have now:

What would be optimal:
