I know that I might be setting myself up for potential disappointment, but I just can't help but speculate. I genuinely believe Nintendo will unveil something StarTropics (ST) related during this year's E3.
This announcement can be a Japanese release of ST, Mike in Smash, or even a third game! Apparently, there was a rumor of Retro Studios working on an ST reboot back in the Wii U days. After reading countless forums, I found that this rumor started as a joke. What if said rumor ended up reaching Nintendo?
The ST series has been dormant for nearly 3 decades, and Nintendo suddenly started to acknowledge it after the reference found in Animal Crossing New Horizons (ACNH)! Here are some examples of this sudden acknowledgment: ACNH reference, pig butt in Luigi's Mansion 3, Nintendo Power Podcast, and Nintendo recommending the game through the News Channel!
Why would Nintendo start to show some interest now!? We also know that Sakurai heard about ST in an interview, and he also has stated that he imports games to Japan, with Super Punch-Out being an example. There's just no way he isn't aware of the series, specifically when you take into account that there are Japanese Nintendo fans who do know about ST.
I once came across a series of tweets in Japanese where Nintendo fans themselves were pointing out Smash's bias towards games released in Japan. They said that it would be fair if Mike joined the game, specifically when considering Lucas's inclusion. Mr. Sakurai has tweeted some images that could be interpreted as references to ST 1 & 2! The two images at the bottom were tweeted by him on a particular day: April 8th. You know what else happened on that particular day; the Gematsu leak happened!
In another interview, Tanabe stated that he would be working on bringing back series in which Takeda worked. We got a Punch-Out reboot for the Wii, then we got a new Pilot Wings for the 3DS! It's also important to note that Next Level Games approached Nintendo for the Punch-Out reboot. If anything, they would probably also approach Nintendo for a new ST.
Around the time of the Nintendo Direct back in February, someone posted an ST leak on the StarTropics subreddit. The leak itself turned out to be fake. However, a certain Twitter user by the name of @HotGirlVideos69 later said that the leak in question was fake, but that an ST reboot has indeed been in the works.
Last year, Nintendo of America (NoA) tweeted a poll in which Zelda fans could what type of Zelda games they prefer (2D or 3D). 3D Zelda won the poll with an outstanding 80% of the vote! ST did more than enough to distinguish itself from Zelda; both series can coexist. I don't know, I just found that poll interesting.
People are still interested in ST, just take a look at how much traction the following tweets got: the ACNH reference, the letter we sent to NoA, the Nintendo Power Podcast, and Devolver Digital's tweet! Anyways, those are my two cents regarding ST speculation.