I’m sure it’s not just me, but I mainly ride on Zwift to race. However, I recognise that structured training will get me better fitness results overall. I haven’t come across a training plan yet that actually swaps out sessions or suggests a race instead of a training ride. It seems like this is something that could be relatively easy to pull into a training plan. i.e. 2 days of structured training and then 1 or 2 sessions are either intervals or a suggested race such as “Go ride a race that is between 20 to 30mins long”.
Yes there are all sorts of reasons why this won’t work (plans are specific and structured for a reason) and I could just as easily swap a session out for a race. But it’d take a bit more of the guesswork out of it and give you something to look forward to when doing over/unders when you know your next session is going to be “fun”.