"For example, each new idea raises incomes by a constant percentage (on average), rather than by a certain number of dollars. This is the standard approach in the quality ladder literature on growth: ideas are proportional improvements in productivity"
I am not convinced they capture efficiency improvements (or externalizations) in this model. Say for example you have an idea for scrubbing sulfur out of coal. It becomes mandated by the EPA, it makes everyone around the coal power plants live 1.5% longer but it doesn't change their income at all. Was it a good idea? Was it an important idea?
We have invested billions in improving efficiency, from solar panels to cars to power plants. How much of that has offset income gains from growth? For example, you add 25% more cars on the road that are 50% more efficient, the net total of gas consumed annually stays the same. So how is that represented in economic GDP ? When you have a video game that is distributed digitally, it costs a fraction of a cent in electricity to "manufacture" and copy to the user. But it represents $1 - $10 in economic activity.
So clearly I've got problems with the definition of a "good" idea :-) I don't like tying it to income improvements. I'd much rather tie to a balance of externalities where the net change is fewer negative externalities. Harder to measure for sure, but ideas that increase efficiency will get as much "goodness" as ideas that increase income.