Actually, Tiptop just dropped a new module that might be a key piece in manipulating modulation signals in the way you need. Have a look at their new MISO module, which is a complex arithmetical mixer intended for CV/mod use. Pairing that with something such as a Xaoc Batumi would make for an incredible and playable combo. Plus, a good complex drone VCO that you should be able to get some mileage out of would be the Synthesis Technology E352 Morphing Terrarium, a dual digital VCO with extensive timbral morph capabilities.