Possibly a better way would be to have bottom and top edges for each climate for existing cliff images? Though even this would mean 2x5x50x65 additional images (although I reckon 20 blending maps could reduce this down to 2x5x7x2+2=142 images to draw). Would need some logic in there about bottom of cliffs as at moment these are all straight and landscape tile in front is just drawn over the cliff behind. You'd be unlikely to be able to see the transitions in 64 paksets but might be able to make interesting effects in 128 or higher. Not sure it would be enough difference to make change worthwhile though, and would only work for straight cliffs, the 90 degree corners would look bad still probably...
Instead of replacing the cliff image, I think it would be better to have two additional images in front of the cliff, one at the top and one at the bottom. Thus, you would be able to show the ground the cliff ends on, and possibly a bit of grass hanging over at the top if suitable. And of course, you could use that for fences on the front of the upper tile.
But I don't think that's in question here. If moving away from ground objects, the thing to look at would be the ground itself.
The ground gets generated by the game, based on the light map, the climate textures, and the blending pattern; and there is only that one ground. Ground objects are essentially a workaround for the lack of variety, I think in most games, there are actually several ground textures instead - Eg. Caesar 3.
I once tried to change the lightmaps to gain more structure in the ground, and it works pretty well. The lightmap does not actually need to be flat, you could even creat cliffs right there - except that it will be textured with the climate texture, and it looks very silly if you build a road on it.
If blending and generating graphics within the program are part of the process, I think it should not be a groundobject, but an advancement on the ground generation itself, allowing multiple different graphics for the same slope to be used, mixing actual graphics of rocks, flowers and mushrooms with the generated slopes from lightmaps, and so on. This would make sure that there is a bit of variation even if you just raised or lowered a bunch of land.
Cliffs designed around water would potentially require not just 65 tiles but also ones taking into account different water positions and start and ends.
By the way, for me it was more about ground variation than an actual cliff project. I worked with ground objects a bit and I think it looks really nice if you have just small variations in each tile, setting there appearence so they appear on every tile. But if you do it, slopes look boring. Maybe having an object that behaves like a tree instead (but does not use up tree slots or grow in forests) would be an easier alternative?