Various theories went around, like Shingo Katori who has illegitimate child rumors, Jun Matsumoto who's rumored to be with Mao Inoue, and Tomoya Nagase and the commoner who looks like Katopan, among others.
And there's also an unconfirmed source on Twitter claiming that the initials are "K" and "O".
There's a possibility that this was based on reports from Osaka's Hapikuru! and Hokkaido's U-gata Terebi last December claiming that SMAP's Tsuyoshi Kusanagi and AKB48's Yuko Oshima are in a relationship.
An insider analyzes the situation:
"If Shuukan Josei really has an exclusive scoop on this then it's either a family member spilled it out, or there's a possibility of a shotgun wedding. They even said that it's so huge that it could make one's hand tremble. They really must have a huge scoop in their possession."
"However, it's pretty rare for these tabloids to have a scoop about a marriage, and not a love affair. They even leaked it out 1 week before the release of their magazine, with some easy hints. This is actually unheard of."
"If one has a huge scoop, it's common knowledge that they'd be very wary so the information will not leak out to other entities. Some people are already calling it a 'solicited article'."
Source: via majide2ch