UPDATE: 06/14/22: Bloomberg reporter Jason Schreier has taken to Reddit to reiterate that the next Assassin's Creed game - codenamed "Rift" - will be set in Baghdad and not the Aztec Empire. Original story as follows:
The setting of the next Assassin’s Creed Valhalla, the Viking Age of the British Isles.
Although Ubisoft continues to add new content to Assassin’s Creed Valhalla in the form of DLC updates, fans are still wondering about the next mainline entry in the franchise - and there is no shortage of rumors about where and when this new chapter could take place. Some leaks have suggested that the next Assassin’s Creed game will be set in Africa, just like 2017’s Assassin’s Creed Origins. Others believe that the new Assassin’s Creed will return to the original game’s Third Crusade time period and possibly see the return of fan-favorite protagonist Altaïr Ibn-La'Ahad. Plenty of other Assassin’s Creed leaks have hinted at settings like Japan, India, or China, but nothing official has been announced from Ubisoft save for the fact the next new Assassin’s Creed game will be unveiled during a special Ubisoft Assassin’s Creed livestream event later this year.
A new leak from noted game reviewer and Twitter ACG has sparked some new discussion about the possible setting of the next Assassin’s Creed game, which they claim is going to be centered around the Aztecs - an ancient Mesoamerican culture that lived in what is now known as central Mexico in the post-classic period from 1300 to 1521. Reddit TheVictor1st shared this leak on r/GamingLeaksAndRumours, where other s have speculated on how Assassin’s Creed could tie real-world historical events like the Spanish Empire’s invasion of the Aztecs in the 1500s to the series’ Assassin-Templar feud. It should be noted that while ACG’s claims have yet to be verified by official sources, he has correctly predicted Ubisoft releases like 2016’s Far Cry Primal.
Aside from this supposed next sequel, there have been reports of several other Assassin’s Creed projects, including a new standalone Assassin's Creed Valhalla spinoff about Basim. This Assassin’s Creed game will reportedly cover Basim’s past in Iraq during the 9th century, but it isn’t confirmed if this will be the next mainline Assassin’s Creed entry or a spin-off. Additionally, Ubisoft has announced Assassin’s Creed Infinity, a live-service title that fans still know little about.
In the meantime, it remains to be seen what the next mainline Assassin’s Creed title will be - and any rumors about where and when it could take place must be taken with a grain of salt until an official announcement is made. That being said, the ancient Aztecs were a rich culture that could provide an interesting change of setting for an Assassin’s Creed game - assuming ACG’s new claims that this will be the backdrop of the next game are indeed accurate.
Source: ACG/Twitter (via u/TheVictor1st/Reddit), Jason Schreier/Reddit