I'm working on a mortgage scenario planning tool. A simple mortgage calculator can give you an estimate as to how much a mortgage payment will be but there are so many other inputs that must be taken into account on a per individual basis. For example someone could input that they want to put 20% down and see the monthly payment amount but there are other fees, such as closing costs, which must be taken in account to determine how much cash on hand a buyer will need to actually transact on the mortgage.

My friend and my lender has built up a giant Google Sheet which he uses with his clients and I've been slowly working to translate the logic in that Sheet into an application. It's been a lot of fun as I've been learning how to replicate the multiplayer aspects of Sheets into a React application.