QGIS 2.18 adds several new expression functions, including functions for angle/distance interpolation.
line_merge: merges a MultiLineString geometry into connected LineStringsboundary: returns a geometry's topological boundary, ie for polygons this is a MultiLineString representing the polygon's ringsangle_at_vertex: returns the average (bisector) angle to a geometry at a specified vertex indexdistance_to_vertex: returns distance along geometry to a specified vertex indexline_interpolate_angle: calculates the angle parallel to a geometry at the specified distance along the geometryline_interpolate_point: returns a point on line at distanceline_locate_point: returns distance along line to nearest line location closest to specified point