Isekai Quartet was a fun experimental series from Kadokawa in which they took four of their biggest Isekai franchises and placed them within a chibi parody like series. Taking place outside of the canon of each of the franchises involved, this mini-series saw each of the franchise characters bouncing off of one another in fun new ways such as comparing how each of the protagonists were brought to their prospective worlds. With a second season on the way, it seems that the door has been opened to many more prospective franchises to get in on the fun.
Isekai Quartet announced that The Rising of the Shield Hero would be transferring into the school with the second season, and the official Twitter account for the series teased that this won't be the only new series coming as more franchises will be revealed as we get closer to its January 2020 release.
The first season ended with the protagonists of Konosuba: God's Blessing on this Wonderful World!, Overlord, Re:Zero -Starting Life in Another World-, and Yojo Senki: Saga of Tanya the Evil, all deciding to enjoy their imposed school lives once they found out they wouldn't be able to escape this new world so easily, so it was teased that a new franchise would be shaking up the dynamic. But now it's clear it won't stop at one.
But what could fans see in the new season? Following all the rules set by the first season, it could be anyone of Kadokawa's light novel turned anime franchises about a character who's transported to a new world. The first four seasons were relatively popular, and the same can be said for Shield Hero, so that's probably the thought process behind the other new additions as well.
So this can include No Game, No Life, but that series might not be considered as a new season of the series has not been confirmed just yet. This also includes newer series like Wise Man's Grandchild, Death March to the Parallel World Rhapsody, or Isekai Cheat Magician, but those series have not hit the same way with fans as the other additions. It's going to be a mystery which series will join the crew next, but either way, it's going to be a fun crossover for fans indeed.
Isekai Quartet is a rewarding crossover for those who have watched through each series, and if it wasn't on your radar before you can currently find the series streaming on Crunchyroll and FunimationNOW. They describe it as such, "The button appeared out of nowhere. There weren’t any signs NOT to push it…so the solution is obvious, right? Is it a trap or the start of something new and exciting? The crews of Re:Zero, Overlord, Konosuba, and The Saga of Tanya the Evil will find out when they go from their world to another and get stuck in…class?! See what adorable chaos they’ll get up to in this collection of shorts!"