ProjectItem.isSelected() - Theoretically a ticket for this one already exists. Please add my vote.
Project.getSelectedItems() - This would return an array of ProjectItems (ProjectItem). This API would allow us to quickly access and loop over the set of items selected by the user, regardless of their location within the Project hierarchy. Perhaps an optional filter ProjectItemType parameter (or array?) could be specified to pre-filter returned values to those of a specified type.
This could, of course, be implemented by the user provided ProjectItem.isSelected() was added. Would prefer an official API as it would theoretically be more performant and would cut down on boilerplate code/reimplementation across projects.