As long as your story doesn't involve the exhausted trope of the main protagonist finding himself in a different world will be accepted for submission and review. And also an emphasis on "himself", as in the protagonist (or protagonists) must be male.
The writing contest is open to anyone, as long as it's in Japanese with the winner to be awarded 300,000 yen (about US$2,600) and the opportunity to have their novel published by Kadokawa.
Novel story submissions have a very limited window of being accepted between June 1 to July 16 with the finalists announced on August 2017. The "winner" will be announced in September.
The giant Japanese publisher hasn't provided any specific reason for their theme to be centered around male protagonists, but the Label 0 editorial team has stated their aim is to push away from the teenaged hero type that is heavily saturating the market of manga and anime, and instead want to focus on promoting stories that appeal to men in their 30's with the theme of "A cool adult way of living.".
Who knows... maybe the publishing industry in Japan is finally realizing that an overwhelming saturation of stories centered around teenage boys over the years is responsible for a great deal of Japanese men in their 30's being shut-ins with the need to constantly want escape through fantasies of their youth years.
But these types of opportunities for aspiring novel authors to submit their works to be considered by a major publisher is because of the trend started by Amazon's Kindle Direct Publishing program that offer similar contests.