Not really an Easter egg but is is awesome, I happened upon this in white Orchard,
Geralt is sent to find the hunter who will lead him to where the Griffins victims were, the hunter tells him he has more or less been exiled from the village because he is a "freak". If Geralt admits he too is a freak, the dialogue the hunter gives goes on to say he and a man named Florian were in love and they were found by some other man who ratted them out. Florian hung himself afterwards.
Anyway, while wandering around, Geralt found a graveyard later, and at the head of the graveyard in a large ostentatious candle lit grave with a sword upon it, I found the name "Florian", and some epitaph was written about going he would find honour, etc. (Can't rmember the wording).
I thought this was such a nice touch, and not completely necessary to the story (devs had no idea I'd happen upon that grave and look at it, it wasn't suggested I go do so!) and yet it made me smile a bit to see it because it reminded me the story is rooted in the game further than in just a short bit of dialogue, it made me feel connected to the story a bit more having heard it and accidentally finding it, made those NOVs seem more real. I also liked that there was a grave for the character who clearly had been persecuted, and that he has been given an honourable burial and might have some peace despite what the village had thought of as "wrong". it was cool of the devs to put it there for the wanderer to happen upon.