Fresh air return and dust
I have been told that a fresh-air inlet, pulling *filtered* outside air into the return, might help reduce dust in the house. The concept is that less outside air will be pulled into the house through random cracks and other openings. That inlet will be actively pumping maybe 75 cfm air into the house, resulting in a tiny positive pressure -- the indoors air will tend to flow outward.
It's hard to say just where is the source of your dust but perhaps this idea will help. And it will probably not hurt your energy bills very much if it tends to displace existing air pulled in thru random openings. Am thinking about doing this on my own house but have not yet.
Best of luck -- P.Student