According to Wall Street Journal's Takashi Mochizuki‏, who is based in Tokyo, Nintendo has “at least one unannounced title for Switch” planned for the next fiscal year, starting in April. Apparently, it’s one "fans would be delighted to know" about. Below is the translated tweet:

“Nintendo said it is preparing at least one unannounced title for Switch in FY19 (April onward) that "fans would be delighted to know." What would that be? Reply your thoughts, please!”

This message was delivered during Nintendo’s latest financial briefing. Mochizuki also mentioned how Nintendo said another unannounced game for the Switch – due to arrive in the next fiscal year as well – would be a “nice fit” for the online service. Mochizuki’s interpretation suggests it's a game you will play with or against other players over the internet.

As there’s very little to work with here, feel free to wildly speculate in the comments below.