During the Government Plane Wreck Expedition mission, you’ll learn the plane belonged to the Fourth Echelon from Splinter Cell, you can open a secret compartment to obtain (and wear) Sam Fisher’s suit by interacting with the computer in the plane, and you can find numerous notes around the plane from “SF,” another nod to Sam Fisher.
If you hire Hurk, one of his possible lines if he falls in battle is “Mr. Stark, I don’t feel so good,” a reference to Avengers: Infinity War.
You can get a costume that makes you look like Rex “Power” Colt from the spin-off Far Cry: Blood Dragon. There are also Blood Dragon eggs in a museum that itself references the director from Far Cry 5.
When you do the For Whom the Bear Tolls treasure hunt, you’ll reach a crypt under the church with a stone sarcophagus. This sarcophagus, when broken into, is revealed to be the tomb of Boomer, the dog from Far Cry 5.
The allied character The Judge, who never removes their mask or speaks, is believed to actually be the protagonist from Far Cry 5.
This is supported by Hurk calling the Judge “Dep,” by Pastor Jerome calling Judge “Deputy” and referencing the end of Far Cry 5, and by notes found in the bunker where your character was at the end of Far Cry 5. One note includes the message, “If I judge as your judge, the judgment is right and just, the judgment is God’s Word. I see now. I am so sorry. Please give me a mask I am afraid,” supporting the idea that they became the masked Judge.
The Five Star Theme Park Expedition mission includes banners about the Rabbids, from Ubisoft’s Raving Rabbids series.