I don’t know how familiar everyone reading this thread may be with the components of Sonic Pi’s architecture, so pardon me if I’m repeating stuff you’re already aware of. Sonic Pi’s audio synthesis engine is SuperCollider, and Sonic Pi’s synths are essentially compiled SuperCollider synth definitions (SynthDefs). As Martin has alluded to, it is possible to build your own synths for Sonic Pi. This is one way to enable new sounds without the need to layer lots of sines/saws/tris in Sonic Pi code.
I personally have been very interested in the idea of making new synths and using them in Sonic Pi, as have others. I have plans for several that I’m going to try to build, and that I would love to have included in the built in collection of synths. Martin is right, if you are interested in exploring synth design, have a look at the links he mentions. Keep in mind that for that synth design document, for new synths we may be moving away from using Overtone as the design tool, and just using SuperCollider directly - so for new synths that make their way into Sonic Pi’s built in collection, they would need to be designed with SuperCollider as mentioned in Compiling your synths using SuperCollider.
(In addition to the links that Martin mentioned, there’s also a little discussion about synth design in this thread: Making New Synths from SuperCollider SynthDefs?).