The quintessential Warrior-Mage, Gish, Fighter-Mage, Battle-Mage, etc is a character trope that is hugely popular among fantasy RPG-ers. The Magus was a ray of light to fans of this character type, the main factor being you could play a warrior-mage right from level 1 instead of waiting until level 2 to multi-class and/or waiting until level 7 to get into the EK PrC.
That's why I will iterate the Magus NEEDS to be in the core book, more than classes like Alchemist, Oracle, etc. If they can all be in that's great, but Magus HAS to be in there.
Since we are on the subject, some of the changes I want the Magus to have are make it less complicated in play, reduce or remove dependence on the standard gimmick-cheese of touch spells like Shocking Grasp (it's almost become mandatory for every build to go this route, making playing it almost a chore) and allow for both a spellbook Magus and also a spontaneous Magus from the get go (should be a built in class choice at level 1 or failing that at least an archtype)
Magus should be about both casting spells while swinging swords, and those who want to buff their swords/melee skills with touch spells can be an option, it should not be the standard default every feels they MUST play to be competitive.