Thank you for reviving this thread with your questions, quite appropriate at this time.
Most preferments are made only with flour water and yeast. Adding Flavours at this stage is not generally beneficial, as they are often Los to the fermentation, but there are sometimes special or unique exceptions.
Preferments can be a a very difference range of processes, depending on flavour and usage outcomes. Most sliced white pan breads are made with a 4 hour sponge. This is usually fermented between 72-80°F. It is typically 50-60% water ratio to flour. This provides a fermented yeasty flavor and improves crumb softness and shelf life.
A poolish has more water added and can be as much as 1:1 four and water. A poolish is generally fermented somewhere between 12 to 18 hours, depending on desired results. Longer produces more lactic acid sour notes that are more toward vinegar flavor.
A Biga is a stiff preferment that is 50% water or less and in general 8-12 hours fermentation. This produces milder lactic acid notes that are referred to as more of a fermented flavor. Often used for sweeter products like Pannetone fruit breads, or even sweet white breads and buns.
Some fruits and vegetable provide unique enzyme activity to enhance or change flavor profiles. You might want to investigate utilizing local produce to create something special.