This workshop paper discusses our mixed-initiative approach that enables designers to rapidly sketch and explore interactive layout designs. Although optimisation methods can attack very complex design problems, their insistence on precise objectives and a point optimum is a poor fit with sketching practices. Typical optimisation tools also fail to incorporate the human in the loop. Sketchplorer is a mixed-initiative sketching tool that uses a real-time layout optimiser. It automatically infers the designer's task to search for both local improvements to the current design and global (radical) alternatives. Using predictive models of sensorimotor performance and perception, it generates suggestions that interactively steer the designer towards more usable and aesthetic layouts without overriding them or demanding extensive input. While this position paper summarises our work from the mixed-initiative perspective, further details can be found in the original publication .