Change field Source Calculation to use Pipelines for fields that support "Source"
If developers were able to tap into and potentially customize the logic for setting the source of an image field, reference field, multi-list field, link field, etc. then much more powerful site-resolving field source logic could be written, as well as other customizations. This is especially important for media/image fields.
The most common use-case I can think of is for multi-site or multi-tenant solutions. The number of templates for these solutions typically increases when fields are given specific sources. This issue is typically mitigated by either making non-specific sources (reducing the quality of the UX in the Sitecore Client), using permissions to control visibility for more specific sources (users that have access to one site but not another would only see the sources they need; this breaks down for global Authors who manage more than one site in the solution with the same account), by moving templates and/or media into the content section of the tree, under the respective site (which is a bit messy and requires additional customization to achieve), or by writing custom field types (which is less maintainable and increases effort to upgrade).
This would be a fantastic feature to help facilitate the creation of site-resolving tokens for field sources that could be added to solutions with a simple processor, or provided by Sitecore natively, should they so choose.