Other Types of Modulation
There are types of tonal modulation, other than the common chord method, that are found in the tonal music of the late 19th and early 20th centuries.  Within these methods, other types of harmonic and non-harmonic elements are used to link one tonal center to the next.  These common elements often involve enharmonic respellings.

These alternative types of modulation are commonly used to modulate to keys that are not closely related.
The fully diminished 7th chord, functioning like viiº7, can be respelled enharmonically to function in four different keys, allowing for a viiº7 to I (or i) progression with four different resolutions.

In each measure of the example below, the viiº7 is just a respelling of the same four pitches.

The German Augmented 6th chord can be respelled enharmonically to function as a V7 in the key of the Neapolitan.  The opposite of that is also a possibility -- any V7 can be respelled to function as a Ger+6.


The Neapolitan chord can also be utilized as a common link between two tonal centers.

There is also the common tone modulation in which a single pitch becomes the link between two tonal centers.  This common tone can also be used/respelled enharmonically.