Yes, Houdini 16 will be supported from the first day. In fact, the plugin code has been updated and is currently working fine with the H16 beta versions, without problems so far.
The new terrain objects could be rendered with Octane if converted to polygon meshes. The booleans should work without problems. The new hair system is not supported yet but will be supported in the near future using the SOP geometry hair mode. Probably support the new infinite ocean procedural will not be possible, because it is a Mantra procedural, hard to support in other engines.
Octane only can work with its own VOP nodes, at least currently. Perhaps new nodes to support some of the Houdini VOPs could be possible once we have the OSL feature to write custom shaders, but it's early to say.
I know that the substance raw textures can be used to build an Octane shader tree compatible with the substance workflow. I'll investigate if the direct conversion of the substance files to these Octane node trees is possible, but I am not sure at all.
At some point, I want to improve the features of the MPlay IPR (picking functions and so on, even perhaps include the IPR in the Houdini render view panel), but the OpenGL IPR available in the Windows and Linux versions is not going to be supported in OSX because unfortunately, the graphics lib used to manage the OpenGL IRP code is no longer supported in OSX since several OSX versions. I need to replace it with another lib, but this is going to be a huge work, so I don't have an ETA for it.
The plugin has all the export options available in Octane, related to alembic files, .orbx scenes, render passes, render layers, etc. But export a complete scene using the After Effects file type is not possible.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini