I want a starship weapon capable of firing in two modes. The first mode is a destructive blast that's equivalent to a Heavy Blaster Cannon. The second mode is a disruptive blast that's equivalent to a Light Ion Cannon. Effectively, the only difference is the Ion quality in disruptive mode. Switching modes would be an Incidental. The weapon would cost 7,500 credits (50% more than either of the weapons it emulates) and could be mounted on Silhouette 4+ (it's a bit larger than either weapon it emulates).

I've considered calling it a plasma/charge/pulse (pick one) cannon, but I'm open to other names, especially if such a weapon already exists in Star Wars.

I'd suggest making it cost the total amount of both the other weapons, together - or perhaps more. Either that or make it use as many hard points as both weapons combined plus one. Why? Well... the weapon you're proposing gives them both weapons at a 25% discount, and if you make it take the same number of hard points as a single weapon normally would, you're making it so it players would be crazy to go with the normal options.

I'd suggest making it cost the total amount of both the other weapons, together - or perhaps more. Either that or make it use as many hard points as both weapons combined plus one. Why? Well... the weapon you're proposing gives them both weapons at a 25% discount, and if you make it take the same number of hard points as a single weapon normally would, you're making it so it players would be crazy to go with the normal options.

Possibly, but in the destructive mode, it's an inferior weapon to Heavy Laser Cannons (which have superior damage, superior range, and an improved critical rating for the same cost). It does appear to be better than the Light Ion Cannon if you don't mind the price increase.

Still, upping the cost to 10,000 credits for the weapon along with a high Rarity isn't out of the question. I'm not going to increase the Hard Points required as a single Hard Point can already mount a Quadruple Heavy Turbolaser without issue, so I figure 1 HP for one weapon system.

I'm not aware of such a weapon existing in the Star Wars EU, but in terms of game mechanics, I'd just simply just add an "Ion Setting" quality to the already existing Heavy Blaster Cannon, and that if functions similar to the Stun Setting quality for hand-held blasters, but without the range band reduction. I'd say a cost of roughly 7500 credits for a dual-purpose weapon would be fair, though it'd probably have a higher rarity value than either the HBC or the Light Ion Cannon would on their own, probably at least a 6.