i thought nico x maki is official...
The franchise creators noticed a trend (before the anime started) among the franchise fans to "ship" certain pairs: NozoEri, RinPana, NicoMaki, various HonoKotoUmi pairs or the trio. So the first season they put that in the opening song scenes*. First season they left things somewhat open while still giving the main trends a bit of extra time. In the 2nd season the Nico and Maki encounters became more common but not romantic, with two exceptions. In one case, they went on a date one night, but it wasn't made a big deal of. On another occasion, Kotori "shipped" them by bringing them a couple's soda with a heart-shaped twin straw. Their PVs were pretty romantic, though. Meanwhile the fandom has branched out, and Maki, a super-popular character to begin with (usually only #2 to Eli), became the character 'shipped with the most other characters. After Nico x Maki, which is indeed the dominant fan-ship of the whole show by most measures**, Maki is often shipped with Rin, Umi, Eli, Honoka and Hanayo.
Nico, in contrast, is also shipped with Nozomi or Nozomi and Eli together, or with Hanayo.
The Love Live! franchise definitely pays close attention to fan trends. If you could get enough fans to support it, Maki x Principal Minami or Nico x Tsubasa would be "official."
*If you notice the lawn photo in the opening of Season 1, they are lying down as if you started by taking the 3rd years, then 2nd years, then 1st years as groups, then yanked Nico out of the 3rd-years and paired her with Maki.
**The Google search trend is like Nico x Maki way up and all other 'ships combined don't add up to a third of their hits. Fan polls go up and down, NicoMaki stays in the top 5 but aren't always on top.