To be fair, we've already seen evidence of three out of four of these. In Shadow of Kyoshi, we had a scene where she showed the ability to bend broken glass out of her neck. In The Avatar and The Firelord, we see Sozin cool down hot lava, specifically manipulating the heat. Lastly, in the Dawn of Yangchen we see Yangchen amplify the sound of her screaming with Airbending. We haven't seen any waterbenders try to bend wood, but we HAVE seen Hama and Katana remove water from live trees in the Puppetmaster, and we've seen Huu bend the water in Vines in The Swamp. I would imagine that because water can be removed from trees, its possible when trees are cut and turned into wood they might very likely contain water within them.