Method declaration line (i.e. "- (void)doSomethingWith:(id)value") could include cross references to known objects in method result and argument types. Probably links should use same color as the rest of the text to avoid visual noise and only show underline if hovered.
Apple's docs have this and it is enormously useful.
I keep having to add @see for parameters just to get the links show up in order to make the docs decently discoverable. Perhaps auto-generate the @see list from method parameters?