Every "main project" I have (or have had) began life as a "side project." Literally. 100% of my non-consulting income (and my consulting income is peanuts) for the past 5 years has been from side projects that stuck and did well enough to go full time.

Given this, it's absolutely imperative I continue to invest time into experimenting and trying out more side projects, because they could well be my main gig in a few years.