I don't necessarily disagree with you here, because your statement actually has a baseline narrative. The arcane half-caster is a threat eliminator. It is a protector and a defender. Which is at least part of the way there. The big question though is how to distinguish this class from the Ranger and the Paladin? The Ranger is an eliminator of threats to the natural world. To nature and those that would ruin it. And they use nature's magic to protect the wilderness and those within it, wearing light armors, skilled in many nature-based abilities for tracking and survival. The Paladin is a heavily-armored knight (with all the acoutrement the identity of 'knight' brings to a person), not necessarily a warrior for the gods but certainly follows the same sorts of moral codes of other divine peoples and protects the world from the evils that manifest just as clerics, priests, and moralistic peoples do. And they swear oaths to these tenets to take on the threats of those that would break those beliefs.edit: all the narrative the baseclass probably needs is 'is specially trained to combine martial abilities with magical powers to deal with specialised threats'
So what threats does this arcane warrior specialize in?
If the Ranger has 'the natural world' and the Paladin has 'good from evil', what does this half-caster focus on? There has to be something more than just "Everything! They protect everything!" Do they protect nobles and those with the money to pay them for their magical protection? Do they deal with the specialized threats to the commonfolk-- the things that go bump in the night and they use their arcane magic to find these magical threats to regular folk? Do they deal specifically with the threats of the Far Realm and other aberrant creatures and events? Give us something that is more than just "essentially a Fighter/Wizard multiclass but with different unique mechanics." Because new mechanics just for the sake of new mechanics is pointless and not something WotC has ever had any inclination to make.