Dragon Slayer Longbow?

17 dmg - magic dmg isnt lost

You could also have +damage vs. good or evil dragons over just a specific type or all dragons. I cannot picture it as much as just damaging all dragons. It does have the old school flavor, but feels wrong to me.

You can also use the time you have left in the campaign to help your decision. If there is only another level or two then go for it and give a big item.