It's nice to see you here in the Community. I'm here to share some information about the Budget vs. Actual report in QuickBooks Online (QBO).
Putting an additional line item that'll show the difference between your income and expense in the Budget vs. Actual report is a great idea! However, this option is currently unavailable in QBO.
I can see how additional options and functionality with this particular report would be helpful for both you and your business and I encourage you to submit feedback. These suggestions are forwarded to the Product Engineers for consideration in future updates. QBO is constantly changing and evolving based largely on the suggestions and requests of users, like yourself.
You can submit this from within QBO by selecting the Gear icon and Feedback or by going to the following link: QuickBooks Online Feature Requests. Please know that I'm going to submit feedback on my side as well for this feature.
That's it! Please know that I'm only a post away if you should have any questions. Take care and have a wonderful weekend.