2.5D effects like occlusion and surrounding, which incorporate 3D blending in 2D designs, enhance visual dynamics and realism. However, it is hard to create 2.5D effects due to complexity of depth perception and the labor-intensive task of realistically integrating different design elements. We introduce DepthScape, a Human-AI collaborative authoring pipeline that places design elements in an implicit 3D space. Leveraging a single-view 3D reconstruction model, DepthScape transforms input images into 3D models, enabling realistic 2.5D effects by positioning 2D elements in the 3D space. To further simplify design process, DepthScape recommends similar designs from our design library, suggests element placement in the 3D space, and provides interface for users to further fine tune the design. We evaluate DepthScape among 9 users and find out that DepthScape is capable of creating sophisticated visual effects, accelerating the authoring process, and also inspiring new ideas.