I think it could be really nice and people that get it, will be happy.
But especially if the game is free, there will be many people, which don't buy the expansion packs atleast not all and when they hear about the secret DLC they probably get mad, because they have to buy everything else to get the "special DLC".
I like the philosophy of CD Project red, that they don't make any pre order benefits or other things, so all players get the same "complete" game. The players don't think they "miss" something and the game get better reputation which also leads to more players.
Maybe you could make an easter egg as reward or an achievement but not a whole gameplay DLC.