This is what I've done too! Not the same exact mechanics but close; in my Pendragon class (which I'll be posting at some point), they start with a Mythic Weapon. I ended up sketching out 33~ new weapon-based spells (including 12 cantrips) and realized that I could really just put the Spellblade (with spellstrike) into a subclass instead of the main class by having the mythic weapon at level 1.One of the things I gave my swordmage class was a rune weapon at 1st level which gives it the arcane property (allows use of intelligence instead of strengh/dexterity for attack and damage) as well as making the weapon magical. It also allows for a small damage bonus chosen from cold, fire, lightning, or thunder damage though I'm instead thinking that they can instead change the weapons damage type to one of those instead of an additional bonus, which probably wouldn't require the weapon to be magical since you can instead just change the damage type.