In the video sequence after the first playable section of the game, if you look closely enough at the side of Sully's plane you will see it is called "Hog Wild".
This is a reference to the first Crash Bandicoot game (also developed by Naughty Dog)...
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Somewhere in the game you'll pick up a treasure called a "Strange Relic" (though I don't exactly know where I found it). This treasure looks like a precursor orb from the Jak games. Since Naughty Dog was the maker of Jak, I guess that's why they had that...
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In the very first cut-scene and level if you look at the wet suits that Nathan and Elena are wearing, anyone who played the previous Naughty Dog game Jak and Daxter 1/2/3 will notice that the wetsuits say 'Ottsel' on them which is the name of the species...
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