I don't think that Eric (aka MC) embodies only the personality of the player (aka an ordinary guy) that much, he has his own personality starting out with quirks, fears etc. As my friend and fan of this novel said: "the MC is an individual character that you can mildly project on to but is still is their own person." And i agree with him)
As for the DLC, I'd like to see it with Spencer and Darius too:) Spencer is my favourite character, I would like to learn more about their (Spencer and Eric) future family life together. And yeah...Especially, i want to see such a dlc after the completion of the Darius route because his route is the most unique and compelling in my opinion and it's so interesting to see dynamics between him and mc:)