I'm not sure if vines are already a thing...I don't see them mentioned in the wiki. I was thinking having the ability to grow vines from the trunk and/or the leaves would be cool, and especially if it could be a defined block ID value so we could use modded vines as well.

Secondarily and less important would be the ability to grow things on the side of the custom tree trunks like cocoa beans for example, but some mods also add stuff like this (like GlowingCocoa and Maiden's Marvelous Materials), so again being able to define a block ID would be awesome.