1) A big one for me. Dedicated mod mappers(scalers) for the mod matrix. Maybe something similar to the way Camel Audios(now Apple) Alchemy does it, so that sources don't just affect destinations in a purely linear fashion.
Say you want a more curved response, or have say the mod wheel assigned to filter cutoff, but for the first 0-64 it does nothing, then from 65-127 THEN it starts to increase the cutoff level exponentially like a curve. Really this is something all flagship synths should have now imho.
2) More LFO's. 3 really isn't enough, I would say at least double that amount to 6 please. Yes of course MSEGs can be looped, but really more LFO's would be great

3) More standard ADSR envelopes, maybe a couple more. Sometimes you don't wanna be faffing around with an MSEG, and just wanna have this modulate something else independently, with release action. And with regards to the MSEGS, have the option for loop mode to be poly as well as the current mono.
4) More per-voice fx possibilities. Taking a nod to the latest Legend HZ, with its per-voice compression, would be cool to see similar stuff in Dune 4. Maybe make both the Distortion FX and the Compressor module in the FX rack the option of per-voice in addition to the existing per-voice effect types thats in the filter section currently.
5) Speaking of FX, have some new ones, like multiband compressor, to tame resonances cycling at different frequency ranges.
6) More powerful ARP/SEQ. I love the insane power of the Arp in Arturia's Pigments, in particular its realtime live playing randomisation functions, and the ability to have independent step length for the different aspects of the sound, note, velocity, gate etc. Sadly Pigments arp is locked to on/off for the entire presets. Dune gives you the fantastic ability to assign Arp to any voice, with 2 of them. If Dune had Pigments advanced arp features as well, it would be the ultimate

7) VA Engine. As well as including Sine wave (instead of having to use WT mode), have some vintage synth variations of all the standard waveshape types just to have that different harmonic characters as a starting point
8 ) More powerful wavetable engine with additional features like phase&ring mod.
9) Pure sampler mode with granular/spectral options
Thats all I can think of for now. Still absolutely loving Dune 3, one of the synths I always reach for first
