alt Hacker NewsSounds like you prefer expenditure or income calculations over production. That makes sense and I think I'd agree.
I'm not sold on tying it back to opportunity cost though. That may require knowing the potential value of work that could have been done instead. It also means that we could view GDP as the potential economic value if everything is optimized, regardless of what is actually produced. That feels wrong to me at first glance but I'd have to really dig into it further to have a more clear argument why.
Production will only change things there if both tasks are carried out as part of the same service, charged as one. Otherwise, there will still be two outputs that nullify each other but both cause GDP to increase. But even then, if you charge someone for a service to dig and fill in holes, that there is no tangible product at the end does not mean there isn't an output that has a price, and that so increases GDP, just the same as, say, performing a dance does not leave a tangible product at the end, but the service still has a price and a value, and paying for it still increases GDP.
With respect to the opportunity cost, the point is not being able to quantity it, but that whether or not the task is productive, because it takes time, it has a cost.