I would find it difficult not to fill that space with a Disting EX or an Ornament and Crime if I were you.
Do you do DIY modules at all? If not, maybe get some supplies and a kit for building a mult or something. If you get some practice with it, you can get other cool cheap DIY stuff like the AI Synthesis Matrix mixer (a great choice for a rack like yours that relies on tried and true modulation and oscillators patched creatively). That'd fill the space for cheap and potentially give you a new skill set.