Classic items for the genre would be steam engines, steam ships and steam powered Zepelin. All classic and useful, but better suited to an operational base.
If we could communicate art on a game map it would have a lot of brass, of course.
Common weapons, such as swords and firearms, would be present. Armor, not so much. I'd limit firearms to mid 1900s, meaning breech loading and with mini-balls and rifling rather than smooth bore, but I'd make six-guns and repeating rifles an oddity, one of those vulgar types of things the "Colonists" in America seem to favor.
Other personal weapons can exist in the game, of course, but if someone wants to create a special rifle it should be ornate, complicated, and require regular tuning and adjustment. The same for anything out of the ordinary, such as a repeating crossbow, dagger-thrower or whatever form of "deathray" you might deign to allow.
The frock coat would replace the cape in most circumstances, though capes might be present as well.
The works of Mary Shelly and Jules Verne were rife with references to capturing or harnessing lightning, galvanic currents (electricity), and how they were generated: "By passing wires through a magnetic field I can produce more than enough power..." (paraphrased from the original 20,000 Leagues Under the Sea.)
The one thing you should absolutely restrict (though not eliminate) is any actual science. Verne's reference to generating electricity ignored the fact that it takes power (torque) to move that wire through the magnetic field.
Radiation was little understood, and therefore very "magical", in story terms: It could be used to explain practically anything, most importantly things it can't actually do.
So think of the rich gentlemen of the Victorian age, in terms of trappings, the "Age of Brass and Steam" in all it's glory, complete with waxed mustache, goggles, and the occasional "Poof" that leaves faces and clothes covered in soot.
The problem with the setting, as an RPG setting, is that everyone wants to be the mad scientist. In Verne's books the mad scientists were mostly the "bad guys". Look at Master of the World as an example. Like Nemo, the main character wanted world peace, but still sought to impose it on others by means of force. Maybe misunderstood, rarely of actual evil nature, but never in a Player Character position.
If I had to do it I'd probably start with a concept like Call of Cthulu (sic) rather than D&D. PCs face monstrous opponents, but have to strive not to become monsters in their own right.