Simple things like that the belief that there are "virtual" particles "popping" in and out of existence and they don't affect the path of "real" particles like photons or electrons.
Simple things like what processes is the matter that exists each side of the slits undergoing in relation to the particles that pass through the slits and how each affects the other.
Simple things like what is a photon or other such particle.
Is the particle/wave duality idea hindering or helping the further understanding of these processes? Is quantum mechanics the best approximation that we have or are there other ideas that would simply the models in use?
I liken it to a programming project that has gone down one path based on a series of assumptions and when these assumptions are found to be incorrect or different from what is actually there, it becomes very difficult to change direction without doing a complete rebuild.
If one looks at the history of investigations over the last 100 years or so, one finds that there are a variety of ideas that never gained any traction at the time of proposal. Yet, today appear to give a handle on some of the puzzles that are being found. These ideas are ignored because they didn't gain traction at the time of first proposal.
Mayhaps, it would be worth spending some time to investigate to see if they have any merit. They may not, but it can't hurt to see if they have.