Plasma weapon in roadmap, blablabla.
Anyway, here an idea i got.
After reading multiple ideas about incendiary weapons, they were generally an addon an existing weapon (incendiary rounds, missiles, or even mines), or a new weapon (which is cool, don't get me wrong).
The idea i got is a short ranged piercing weapon that pierce only one physical barrier (only one shield, or multiple layers of armor without gaps. a gap being anything other than armor) and set a fire in a straight line in the ship.
It's very fast, like a railgun, and may look like an ion (the weapon, not the beam). But it's not a continuous beam and does very little damage itself.
Yes, i expect that weapon plus some disruptors to be a death sentence to ships using only shields.