You could always do what my DM is currently doing. At first mythic tier, we were given the path ability "Legendary item" but we had to choose a weapon. But as we gain tiers, our weapon follows suit, without having to use the path ability again. At 4th tier, it becomes a minor artifact and 7th tier, a major one. Honestly each tier could grant a +1 bonus to the sword, allowing the character to apply that bonus directly to the sword, choose an enchantment to add to the sword, or save it towards another more expensive enchantment (+2 or higher, like Holy). Just remember if you follow that progression to tone down the amount of treasure gained as it won't be going towards their primary weapon.
