Kieran, while reading your question all my hairs stand up!!! Use Cases on a project charter, I am sure that is not unheard, but totally orthodox :).
Why would you want to include these detail at this level?
* * *
On your other comment, I see fine that you have implementation approach. Once you have a few PMs in your firm, then you may think to standardize the approach, eliminating the need to establish the approach in every project.
To answer your questions I will put the following diagrams or formulas in a project charter:
* ROI -- Investment y-axis - Time x-axis - Return y-axis another color
* Project Cash flow estimate
* Project total investment over time