If a zone is already burning, what happens if the opponent attacks that zone on a subsequent turn? Like, if the opponent wants to destroy a unit in a burning zone. Can the defender choose not to defend, so that the damage is assigned to the already destroyed capital?

Yes.

Basically now there is only two things I can think of, which makes attacking a burning zone a valid tactic :

- units with the "Scout" keyword

- the "Animosity" tactic card

Cain_hu said:

Also Malus Darkblade.

Oh, and you cannot damage a burning zone if I remember correctly. Another ruling handed down by the dev team.