Check out this approach to isometric pathfinding
facebook.com/photo.php
From Kenney.nl
1. First I draw a polygon where the user can walk. This would be the ground where no obstacles are.
2. Then the game automatically generates a tile map over the polygon, whenever a tile touches a polygon it's marked as a walkable tile.
3. Using the A* algorithm I calculate the path from the player to the point he has to walk to. Diagonals are a bit more 'expensive' to walk on, read more about A* here: en.wikipedia.org/wiki/A*_search_algorithm
4. Now comes the best part: I shoot a raycast vector from the first point in the path to the next, if the raycast doesn't hit the edge of the polygon then it goes to the next point...and the next, and the next. Until it hits an edge of the polygon, then it marks a point back as a straight line. It does this for all poins, resulting in a smooth, straight path which looks very natural and would be the path we humans would take.
5. Hide everything for the user and make the player walk along the path.