Thinking about the Handmaidens in WHF, I ended up reading about the Sisters of Averlorn, from whom most Handmaidens are chosen. The sisters (and so presumably the Handmaidens) use magical flaming bows . . . It made me think that this would be an amazing addition for Kerillian, tied specifically to the Handmaiden career. Being a magic bow, it would have infinite ammo but use the overheat mechanic.
I think it would be thematic and just really fun for Handmaiden, while looking extremely cool. I could imagine it shooting weaker, armor-piercing shots on light attacks, and like a Sienna staff you can charge with alt-fire until it fires a powerful arrow with a slight explosion. Or perhaps it could have very good piercing; I dunno, the details could be debated. But no matter what, a bow made of literal magic flame would just be pretty amazing.