Figure it out for a top view first, then you can transform the positions over to isometric. My idea is to have the terrain be a grid of points at various elevations and then you just find where the player is on the grid and interpolate between the nearby elevations to get the in between elevation.
At least that's the rough idea. Here's a proof of concept of the idea:
https://dl.dropboxusercontent.com/u/542 ... rrain.capx
Half of it is just generating the terrain.
Now another idea could be to layout the level in the editor and put a bunch of invisible sprites over the terrain and see what the player's feet is overlapping and somehow alter the speed when walking over slopes. Maybe something like this:
https://dl.dropboxusercontent.com/u/542 ... imple.capx