The capacity to do this with a number of either degrees or radians a second. The blend functionality is a biggie because its something I seriously think UE4 has needed from the beginning and doesn’t have. A huge thing a lot of people want is the ability to put in a directional vector, an up vector and have it spit out either a rotation or a quaternion (which in the former is easy enough to do just using vector composition). If you did one with a look at rotation as the forward vector and then the arbitrary up vector, you’d be saving a lot of people huge headaches since the most common problem when working with advanced math in UE4 is just aligning an object to a surface normal (typically a character). Not hard but its something a lot of people spend a while scratching their heads trying to solve.
Its not part of quats but the only other thing I can possibly think of is a way to rotate an object from a center which is not its default centre – for example, an orbit. This would need to be done in a way which wouldn’t violate the **** out of basic collision math provided by UE4’s existing systems and may in itself be an idea for a plugin.