He said that he's *not* trying to produce sides on the hexies.
I'm not sure that what you're trying to do is entirely possible - because if you force one hex to be 'flat' so that all its verts are coplanar, then the surrounding hexies which share more than one vertex will have very few options for being on different planes. It seems like somehow it can be done, but the variety of geometry won't be very awesome.
I think the best choice would be to make the center vertex of each hex as 'centered' as possible - averaging the surrounding verts to produce its vertical position. But making them literally flat/coplanar would be hard what-with the neighboring hexies connected to it, which the flat rule would have to somehow apply to as well.
Very nice work thus far, interesting
Ah, sorry, I missed that. Typically when someone posts an image, they are showing what they want to do, not what they don't want to do.
Still, what I said stands, as you stated. With a heightmap, there are always going to be some slopes, as long as some hexes are raised and some are lowered. The hexes "in between" will be ugly. I still stand with my assertion that the best bet would be to ditch the heightmap and just use straight meshes.
Edit: Reading the OP more closely, I'm not sure there really is a way to do what you want. It seems like you don't really have a clear vision of what you want. It might help if you somehow manually constructed a mock-up of what you have in mind, since that might help you to visualize solutions to how to implement it.
If you are wanting to have a mix of rolling height-map terrain and cliffs both, then typically this is implemented using a combination of height-map for the rolling terrain, and meshes for the cliffs that are placed on the terrain to cover the "ugly" hexes where hexes slope from high to low.