Are you planning to allow choosing how many workers each app has?
Let's say I want to build a custom plan with 5 small apps and 1 big app. For the small ones, one worker is enough, but for the big one I need 10 workers.
The only way to achieve this now is to create several separate PythonAnywhere accounts OR assign 10 workers to each app, what results in much higher price and a huge waste of resources.