@zaffy2005 The scion classes weapon bonus skills are main class only, the multipliers are too high for these to be usable as subclasses and they exist to make up for the scion classes not being able to have subclasses.
The other scion classes after Hero were balanced in a way that they have another buff that works for subclasses though.
Using Phantom again as an example, Phantom's weapon bonus skills have 165% multipliers which only work for PH weapons when using PH as a main class, but they also have an All Attack Bonus skill that works as a subclass.
This All Attack Bonus skill has different multipliers for each type of attack (145% melee power, 130% ranged power, 135% technic power, 135% pet power) and also other conditional skills that also increase power depending on certain conditions.
Hero's counterpart of these all attack bonus skills and other conditional power boosting skills is Heroic Boost, but due to how high its maxed multiplier is, it likely would be too overpowered when used as a subclass (especially now that it has been buffed to a 80% bonus on the JP version, instead of 60%).
For Hero to be usable as a subclass, it would probably need an overhaul to Heroic Boost, like putting a part of its power on a Hero version of All Attack Bonus.
It also would need additional unique skills to be able to compete with the other scions as a subclass, something more than just power multipliers (Phantom has PP and focus boosting skills, Etoile has huge damage resistances, and Luster has an unique mechanic that works both as main or subclass, which has different effects as the player does alot of hits, and also a skill that boosts the power of gunblades).
Though, in the end it doesn't really matter much, since while PSO2 is gonna remain accessible, SEGA is gonna be focusing on NGS starting next year, which will have its own different class mechanics from the current PSO2 (a few of the improvements to Hunter/Ranger/Force that were shown on TGS were inspired by the scion classes).