A lot of issues are complex, requiring a lot of smaller steps to get to the end result. FS should model that in the following way:
Any issue can enable a “checkpoints” feature by adding other issues to a list of checkpoints. This represents a step-by-step overview of the steps necessary to achieve the final goal. These checkpoints, as separate issues, can be sponsored individually.
What makes this cool is that the first issue page, the one for the big issue at the top of the heirarchy, displays the completion status of all the checkpoints, and for its bounty, displays a total of its own funding plus that of all its checkpoints. So if I sponsor “Fix automatic font retrieval” for $20, and that issue is listed as a checkpoint for both “Interface upgrade” and “Get rid of all WTFPL-licensed code”, the total sponsorship for all three will go up by $20 (though it is the same $20).
Splitting up a big issue's bounty among its component steps. So if the big issue has its own bounty of $300, not counting checkpoints, then that $300 is split up between the component issues equally, or proportional according to their own sponsorship. This would be a complex undertaking though, and probably not too good an idea for that reason alone.