I understand why dead wood has to be a creature--yes it's kinda weird but if it were a permanent, how would you ignite it? I think both cards should reflect more physically though. Some suggestions:
Dead wood cannot be lighted if it is underwater
Wildfire does not affect creatures underwater
Wildfire is extinguished if it is underwater
(future) Airborne creatures have an 80% chance to escape damage from wildfires
(maybe) Increase wildfire's damage but limit it's AoE
(EDIT: By underwater I mean within flooding's AoE)
If there ever is a chain lightning or fire mortar implemented, a fractal dead wood deck would be really cool though. Kinda like how this idea introduces another non-standalone card, like aflatoxin does with malignant cell.