I've made the tops of unwalkable tiles black. Works well in the caves, but I don't think it works with lava:
Cave.png gets that change, the not-so-straight tile for the northern fringe of unwalkable areas, and some rocks in water:
The ramps have some semi-transparent pixels (assuming that's OK). Seems to add a little when they're over water.
I'm not happy with the eastwards and westwards ramps yet. Eastwards and westwards will be an annoyance to mappers no matter what I do.
Northwards and southwards are better, both in looks and ease of use.
If you look at the "+" shape at the top right of this set, the ramps fit on to it. Eastwards and westwards overlap by one tile; the southwards ramp doesn't overlap at all, and makes the tiles south of the crossing in to parts of the ramp.
Updated ramp test map, and graphics sources: http://www.s.cotton.clara.co.uk/gallery ... 01.tar.bz2