The Dashboard could take many forms. I have seen some very creative ideas using roll-up calculations in Goals to generate the data, and then use a View+chart to display your results. SSRS is another possibility, but it suffers the same problem that Goals have in that it has limitations on the number of records it can work with.
There is a free add-on to Excel called Power Query which you might find useful because it lets Excel query data from Customer Engagement (CE), and then your dashboard is a tab that represents the data with charts or tables. My favorite dashboard tool these days is Power BI because it lets user interact with the data, as a developer you have more control over the results, and fewer restrictions on the number of records you can work with. It also allows you to embed Power BI into a CE dashboard so it appears that you never leave CE.
If you want to do forecasting, the Probability field on the Opportunity is used as a multiplier against the Estimated Revenue, and when used in the aggregate, is called the Expected Value. It commonly used to forecast revenue for a sales pipeline report.
If you are a fan of predictive models, you could use the current years sales, adjustments for seasonality, and current growth curve to predict future sales. I just finished taking a masters class on this subject and would be interested in helping you out.