Even splits are rarely, if ever, fair. Neither are fixed splits, which is when equity is allocated in advance of work being done. These are common practices, however. So common, in fact, that people think they are good ideas-- they are not.
All of the factors you described can and should be accounted for in a dynamic split. Each contribution has a fair market value which is the price at which you could acquire the contribution for cash. So, a person's share should be equal to the fair market value of their contributions (plus a risk multiplier) divided by the fair market value of all contributions made by all contributors. In a dynamic split the allocations adjust as additional contributions are made. It is a straightforward method that guarantees that each person gets what they deserve. Here is a link to a video explaining how it works: http://www.slicingpie.com/equity-splits-at-stanford-university/
You also need to agree, in advance, to what happens to shares when someone leaves the company. There are different reasons someone could leave which will impact their rights.
I've written a book on this subject, called Slicing Pie, I would be happy to send you a copy if you contact me through SlicingPie.com