I was talking with one of my friends around this idea. I was a heavy player for The Division and they had some kind of events to grind for specific gear and customization items that still today are on repeat, allowing new users to get the same as the veterans who have been since the start of the game.

For Nintendo it could easy too, some ideas I came up:

"Future Splatfests"

Yoshi vs Kirby;

Best Champion on Zelda BOTW in 3 splatfests during 3 weekends a lá Sario Characters in JP Splatfests;

2 characters for the last DLC of Smash (this one could see a increase of players interested in smash who never played Splatoon;

GBA vs DS;

Xenoblade 2 vs Xenoblade 1;

Sword vs Shield (Pokemon related);

They could even team up with brands, as they done in JP with Nike, Uni Qlo, MAC Donald's, Pokky and the Japanese Sea Research Institute.

Or even better all the loose side splatfests against the winning side, as a confirm all or deny all changes (since Splatfest results are definitive and affect the world where Squids and Octos live).

Of course the good of splatfests were the Shifty Stations maps but even if they were the same on repeat, I think users would not mind, specially new users still picking up the game who will never play a Splatfest.

Or repeat old ones, as they done with the Ketchup vs Mostard and Trick vs Treat.

Since no news about Splatoon 3 are on the horizon it would be a good way of keeping the game alive and with content until the next one.

That is just my stupid take on this.