Yang Xiao Long's motorcycle from RWBY can be founded parked in front of a shady nightclub in a more dilapidated town. Rideable.

Rebecca's guns from Cyberpunk 2077: Edgerunners, can be found in an alleyway of the same town. Functionable, but killing someone with them just sends them back to spawn.

SCP-1155 can be found on a wall in the same town, with blood nearby. Approaching it means death and you are brought back to spawn. It moves around that area, as well, but in a predictable fashion, since pulling it off 100% to the lore is impossible.

Some graffiti depicting a member of the Qu species from All Tomorrows, in an unknown location.

A copy of the Lusty Argonian Maid in a random library, somewhere. Behind the counter. Reference to The Elder Scrolls. Can be opened, but only has a few pages, due to the size of books in TES games in general.

A room found in one house that has a grody-looking VR headset + a computer turned on to seemingly what looks like VRChat. Unplugging the headset sends you back to spawn, with the headset going back in place.

An obscure location can lead to a secret level resembling a copy of Level 0 in The Backrooms. May or may not need to look into this a bit, since there are several recreations of such.

One of the parks, which is based around a beach, has a familiar looking pineapple under the sea... (SpongeBob)

One of the weapons displayed in the museum is an eerie white-colored baseball bat covered in dried blood (reference to OFF.) The Batter's signature theme song will play if it is taken out and wielded, albeit only to the player with it. Would have to make sure I can legally even do this.

Very slim chance of seeing a large crystalline entity in the voids of space behind the planet if using the observatory. Reference to Stellaris, with their crystalline entities.

Probably several dumb memes hidden across the world. Undecided.

A very, very small chance on every Wednesday for the swamp and jungle-themed parks to be full of Budgett's frogs, while a chorus of "It is Wednesday my dudes" can be heard in the distance.

A house that has a kitchen that looks like it was ravaged by HowToBasic, complete with eggs everywhere, both cracked and uncracked. All the TVs/equivalent have uh... lewd images and videos of eggs on them.

Another item in the museum is a golden gun. However, due to the nature of the materials used, this gun is clunky and unusable, and firing pushes you back a bit from recoil. (ITS FUCKING JAMES BOND)

Bottles of dandelion and osmanthus wine can be seen in the bar (vaguely referencing two characters from Genshin Impact)