A Japanese trademark uncovered by Siliconera
offers a clue as to what Nintendo may call the 3DS Expansion Slide Pad
outside of Japan -- even though the peripheral has yet to be announced for release outside of Japan. The fact that there even is such a name suggests that a western release is likely (and unsurprising).
The trademark refers to the "Nintendo 3DS Circle Pad Pro," a name reminiscent of the "Classic Controller Pro
." That makes sense, as the peripherals are similar: they're both controllers with comfortable grips, and designed almost exclusively for use with a Monster Hunter