I have a player who wants to make a Bladebound Magus/Paladin multiclass. I've been looking over the rules, and there seem to be some balance issues as it is; specifically, that if he is using the Black Blade as his primary weapon, and he only takes 8 levels in Magus, he'll have a +2 sword as his primary weapon, plus whatever bonuses from his Arcane Pool/Divine Bond features (they seem to be comparable in the amount of bonuses they give).
So, the way I see it, I have two options. Allow him to add enchantments the Black Blade, something which not only is dubious in the rules (primarily due to item ego), and doesn't fit with *why* the blades work in my world, or have the weapon scale up based on *Character* level, rather than his Magus level.
The closest I've come to an answer is to have the Black Blade's stats (but not the specials), increase based on Character Level. However, I'd like to get opinions from some other people, as I'm not the most experienced gamer ever. My questions are as follows:
1) Are there any major balance issues with this rule, particularly in how it relates to a Bladebound Magus/Paladin multiclass?
2) How would you change the rule so that a Bladebound Magus can multiclass into another class that has a low Will Save without allowing the Black Blade to control him whenever they disagreed?
3) Would you extend the increase to the special abilities of the Black Blade, such as Teleport Blade and Transfer Arcana, and if so, why?
4) Are there any other solutions that I am missing?
Any comments or advice would be greatly appreciated.