Platinum was announced May 2008. May CoroCoro only showed Giratina's alternate forme. June issue showed the starters, updated designs of Lucas and Dawn, Looker, Charon, region overview, and Sky-forme Shaymin.
HGSS was announced May 2009. It only showed the logo. But during that month, Smash(?) gave us a small teaser trailer of HGSS. Also during that month, CoroCoro revealed Ethan and Lyra, starters, and preview of the region.
BW was announced April 2010. April issue of CoroCoro only contained screenshots of the Unova region. May issue showed Hilbert and Hilda, as well as the starters. Also during that month, Mascot legends were revealed, but outside of CoroCoro.
B2W2 was announced February 2012 and it only showed the mascot legends. March issue gave nothing. April gaves us a blowout; it showed the Mascot legends, Nate and ROsa, Hugh, Colress, Marlon, Roxie, and the region overview.
XY was revealed at a Direct in January 2013, that already showed the main characters, starters, legends, and the region. February issue only showed Sylveon. March showed nothing. April showed Mega Mewtwo Y. By May, they showcased new Pokemon (Pancham, Fletchling, Gogoat, Helioptile) along with starters and mascot Legends. The official site also updated for the first time with all the info they have revealed since the Direct in January.
ORAS was announced May 2014, with a teaser trailer that just showed the boxart and mascot legends. June issue contained the Mega Evos of starters and Mega Diancie, Brenday and May, Steven, Team Aqua and Magma.
SM was announced February 2016, with a teaser trailer showing concept art, and a wire-frame of a bird Pokemon. March gave nothing. April has blank boxarts and Magearna's typing.