Steamfantasy. C'mon. Were you really expecting something else, from me?
Full on Victoriana. Urban Adventures, Ingenuity Systems, Airships and Flying Combat...
Could also explore different Steampunk themes. And there -are- different steampunk themes people don't really think about too terribly much.
Gaslamp Romance: Urban Adventures, Wealth and Power, New Technology, Polite Society. The pains and changes to society through advancement and the inevitable backlash against that advancement. Fear of the Future collides with Horrors of the Past. This is where your Jekyll and Hyde, Phantom of the Opera, and Penny Dreadfuls happen. But it's also where political intrigues, secret societies, and detectives seeking terrible truths exist.
Weird West: Wide open Prairies, Small Towns, Edge of Expansion. Where modern technology is crashing into ancient myth. Where old cultures and new cultures clash and colonialist intentions can be examined through a lens of allegory. Where Legends roam and fall and rise anew. Folk Tales and Ghost Stories told around the campfire.
Adventurism: King Solomon's Mines, Indiana Jones, the Mummy franchise with Brendan Frasier at his height. Safari and Exploration, pressing the edges of the map to find out what lies beyond the frayed edge. Airships and Pirates, Lost Worlds, Insane Depths of the Sea or the World itself.
Because the core of Steampunk, the most central tenet of that idea, that genre, is that humans are inventive. We may not be the strongest or the fastest, may not even be the smartest. But we are -creative-. We build new devices, we formulate complex plans, and we overcome strange forces and terrible plots by innovating our way around the obstacle to achieve our goal.
But it's also a double edged sword. In fighting against cosmic forces with our ingenuity, we can sometimes create terrible things because we've transgressed. Whether it's waking the Mummy, creating Frankenstein, or far worse things... we can become our own monsters.
I'd love to see D&D give a Steampunk world the same level of narrative attention that they're applying to Ravenloft, now.