Mix-and-match attributes is a pretty good idea, though, although I do agree that taking both boss and Biometal attributes into one character would be a bit too broken. You could do Head (touch-screen abilities), Leg (mobility), and Arm (weapons) attributes of each of the Big Four Biometals, but possibly toning it down as opposed for a full Double Rock On (maybe modifying normal attacks but not charged). 'course for H's sake that would require actual dash exhaust from the feet, something Inti apparently doesn't believe in. And you'd need some sort of mobility upgrade for F, maybe a dash-attack similar to Atlas's.
Something like that could probably work as an upgrade to Model X.
I just really want to see them utilize all protagonists (Vent, Aile, Ashe, and Grey), although this will require a new Biometal (Craft works fine for me).
Two characters, if one has Model X, should be able to activate Double Rock On together, but some difference shown for the affinity of each character. Let's use the ol' Aile/Model X and Vent/Model Z scenario. They Double, Vent's Model ZX is the traditional "saber with buster-mode" that we're familiar with, while Aile is the other way around. An arm-mounted buster with a Blade Armor-esque sword mode. Vent would be better with the saber (longer range?) and Aile better with the buster (spiral-shots on both Double Charge shots?).
Likewise, they could do Model AX, one using an arm-mounted buster with a stronger charge attack, and the other pistols with a wider Homing Shot range and better rapid-fire.
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