I would like to see a mod that introduces roots underneath trees. They could be crafted into planks just like logs. They could also spawn in various sizes deep underground. There have been too many times Ive been mining along and ran out of wood and had to travel all the way back up to cut down a tree. You could also implement oil that could be found underground collected in buckets and it could be used as a fuel source like coal. Even better you could refine it into pitch that when dumped can be set alight and would burn a long time like netherack. For advanced crafting techniques you can make a cauldron fill it with pitch and light it on fire. When clicked on one side it pours out the other for placement on walls to passively kill mobs threatening your walls (This could be a finite source making you to fill it again once clicked if you want to use again). You could also create ancient roots deep below that drop amber and exp. orbs. Amber could be a material in tools and armor that is stronger than iron but weaker than diamond.
Buildcraft has oil. Several mods include amber as a material. Lava would do the same thing as pitch, even though you have to be careful what kinds of things you set it next to. And some of the trees in Twilight Forest generate roots. However, couldn't you avoid a mod just by making sure you have a stack of wood in your inventory before going mining?