small budget app for personal use

I'm making myself a little thing to help me budget my finances. What type of specific budgeting help would you find cool/helpful?

So far, it will have a few main numbers: My pay, tax taken out, and what's left over (what's left over being the main number).

Then, money made on the side can be added (but will exist under a different set of budgeting goals).

Then, a pie chart where I can define percentages for categories of finances: (Rent/Mortgage, Groceries, Entertainment, Utilities, Insurance, Car Payment, Eating Out, Entertainment, etc.)

Then, a second pie chart that gets filled, as a month progresses. I'll go into the app and add what I am spending and in which areas. So, the first pie chart is like a goal-setter. The second would be what is actually happening. So, let's say I have to go over my goal when it comes to eating out … then I'll see that in the pie chart, and I can begin to shift my budget in other areas to keep everything controlled.

You can add what you spend and in which categories — all of which have spending goals set for them, then if something goes over, it will try to help you offset it in another area. Like: "Hey, yea you spent way too much buying lunches every day, might I suggest trying not to go over $200 when you go to the grocery store next time?"

I also want to be able to tinker with my numbers and let it show me what would happen – under those new variables – across a span of time. Like: "Show me what happens if I lower my spending goal in these two areas across a span of 6 months, 9 months, and a year."

What would you say you wish you had control over for your budgeting?
What other features would be helpful?
What am I missing for this thing to just be cool AF?
Probably will open source it. It isn't anything special at all, just something I want for myself.