Typically in the bad old days up to three quadrature osc would be summed - you could do more but it tends to be more garbage than value. Each osc has a sine/cos and often a inverse sine output. (Substituting the inverse sine for the sine reverses the direction the "circle" is drawn and so for example changes a triangle (made by two quadrature osc at the right relative frequency and gain) to a cycloid with a single inside loop.) You could run VCOs in series for a frequency modulation effect, but you could do that with a summing amp on the input and just routing the output of one of the paralleled quadrature osc to the VCO input - or in the case of a the phase rom mentioned - you could use a unipolar or bipolar (or other shape) version of the input clock with a divider to get it into the frequency range of the output. The osc can also be amplitude modulated for some interesting effects. Next an analog multiplier on the output of those summing amps will support spirals and diamond spirals. (or a place to inject audio mod.) Inject an offset and you get swept spirals. Four analog multipliers and two summing amps can perform z axis rotation. At some point you'll want to switch sources either mechanically or using analog switches and this is where you need to consider just how you're going to deal with gain and offsets generated by your different signal paths…
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso