Have a voting contest for /ourgirl/ on all boards.
E.g:
https://boards.4chan.org/a/ourgirl
https://boards.4chan.org/b/ourgirl
Each board would have their own theme (purple vs green) depending on which girl has the majority vote.

Tell them that the outcome will decide who becomes 4chan's favourite mascot.
[spoiler]If Sachiko wins, we'll just tell them "April Fool"[/spoiler]