As I see it, complexity often starts with limitation rather than bloating your patch. You can tie a few pretty snarled tangles with a single string, so the real challenge begins when attempting to do more with less.
Try getting big sounds with as little as possible and your patches will become inherently complex really quickly. Polyphonic modulation often is a good tool.
Emulations of Rob Hordijk’s Blippoo Box can be rewarding, too, for example: Blippoo Box by Rob Hordijk.
Also, Krell patches are quite fun to study. A few links compiled at: The ever-changing generative patch challenge? - #21 by Alphagem-O,
… and a more recent patch: