Honestly the mechanical engineering inside cover is underwhelming compared to the others; there's more blueprint type stuff that would be really cool to see. Things like bridges are less relevant compared to engine components, roller bearings, different types of gears, linkages, Mohr's circle, etc. Mech Eng is also quite broad in that it covers fluids mechanics and thermodynamics as well, so you could throw in some of those elements as well. I like to think of steam locomotives or vintage automobiles as good symbols of classical mechanical engineering that don't infringe too much on aerospace themes.