Xavier, and plans to move to the STM32F7 series? Since it sounds you're already maxing out the CPU on the Cortex M-4. I would love to see delay based effects ( reverb would be nice, but probably too much to ask without a dedicated DSP). Since the PreenFM2 is multi-timbral, I find it challenging to layer outboard effects on it, since I would want add a delay to I1, a bit of stereo imaging to I2 and nothing to I3-I4 for example. I've played with the idea of using L/R output as a Dry/SendFX level instead, and use panning to control the amount of effect instead of the stereo position, but that's also a trade-off and only allows for a single effect.
The PreenFM2 is sitting next to the MicroMonsta, and the FX section really allows the sound to expand on that little synth. I would love to see a similar approach added to the Preen.