The only HD remake of a game that I've thoroughly enjoyed was Black Mesa for Half-Life, and even now it isn't completely finished yet. Still waiting for them to release the last half, but where they left it off (jumping into the portal into Xen at the Lambda Complex) was a solid ending with enough closure, so even as a standalone game, while short, it's remarkably good.
If any of you are fans of FPS games, then I would suggest you get it. My only criticism is that the marine grunts seems to be ridiculously overpowered, but I feel like that's only because I've been thoroughly spoiled by the forgiving enemy NPCs of modern FPS games.
I think with Black Mesa it is a little different, the original game was more than eighteen years old, and though it was pretty impressive and advanced for its time and set a whole new standard for story driven FPS, the genre has made quite some advances since. (like physics for example)
That was now implemented but the designers also decided to completely overhaul the levels and add new features and alternative paths for the player, making it rewarding for both the old players and new players who are interested in trying out Half Life 1 but previously for some reason decided not to.
It was much more than an HD update, it was a remake that expanded on the original and not retold it in a completely different way.
If we would use Black Mesa as an example, Fallout 3 HD would not only have to have better graphics, but perhaps an expanded main campaign, additional single quests, more filled in locations,features taken from FNV such as customizable weapons and gear, increased variety of weapons, various types of munition, expanded work and reloading benches.
And if features from Fallout 4 were used Fallout 3 HD would have to be expanded to an even larger degree that it would become almost a full AAA project.
I think it will be just a reskin, like
MercenarySnake already wrote, for console users who can not update their old game with mods and better textures like PC users can.