Dildo: A scaled-up headphone plug, about six inches long. Could feature tight-fitting alternating bands of stainless steel and plastic/silicone, or simply solid plastic/silicone with a metallic finish.
Sleeve: A scaled-up silicone headphone jack, about six inches deep.
Not sure if the latter version
would be popular, but the former might be, particularly among hipsters.
An additional feature might be to make each device vibrate to the bass line of a song only when completely 'plugged in.'