Brock returns and interact with Cilan.
Newtype: (airing on October 3, apparently)
Dent and Takeshi! The Angered Gyarados/Gyarados's Outrage (Imperial Rage?)!!
(since げきりん is also Japanese for the move outrage... I could be wrong though)
Iwane is directing it.
PS: 2ch labels it as BW "外伝" (Gaiden/chronicles) and calls BW142 as the final main episode. So, it could be a side story thing. But no official confirmation, whatsoever.