Generally speaking, if it is something that has a long-term effect on my character sheet, then it should be in the Player's Handbook. So, that includes races, classes, skills, feats, spells, alternate class features, alternate race features, familiars and animal companions, equipment, and yes, magic items. As a player, I want one book to access everything that I will need to run my character.
At a $50 price point, I'd like to see all the races and classes from the first 1e, 2e, 3e, and 4e Player's Handbooks. That includes Assassins, and Gnomes, and Warforged, and Warlords (or close approximations thereof). I'd like to see a lot of options to customize the character.
Mostly, what I want is one book. I don't want to have to access a Dungeon Master's Guide if I find a magic item. I don't want to have to access a Monster Manual if I buy a horse. For certain, corner case items, like a griffon, sure, Monster Manual; but for the most part one book.