Density - it says here - creates "layers of sound from a single input", and promises to turn single voices into choirs and violins into entire string sections. I mean, lower your expectations from that obviously, but it is pretty reasonably priced (£74 intro price here). There are demos on the website that are, imo, more successful than some of the ones in the video below - the solo violin there was so dynamic in the phrasing that the end result was slightly hideous, so I think it will be very material dependent:
It's clearly based on their technology in the full Sound Particles plugin, so it's a little disappointing to see owners of that don't get any form of discount. There is however a 30 day free trial.
https://www.soundparticles.com/products/density