If they're smart, they'll run with the whole "myth right below our feet" theme they've been toying with. That place where Arthur got the Trident was Skartaris, without question. And here's what you could do with that:
Miles below the Earth's surface, there is either a "lost land" or it ceases being earth altogether and is its own pocket dimension, bordering other "underworlds." Now in the comics, people of no strong faith to the contrary often end up in the afterlifes of mythologies.they've interacted with. So if Steve died stopping Ares's plot, that would send him to the Elysian Fields. Now imagine Steve kept trying to leave and was eventually punished - think Sisyphus, just pointless endless toil. Use this as a subtle reference/homage to Conan.
Now let him escape across.the river Styx only to end up stranded in Skartaris. Now you have Steve Trevor as the Warlord, complete with a shade's supernatural revolver(the shade of the one in his hand when he died) that's never empty. Diana discovers him in her quest to replace her lost sword as she realizes magic is indeed re-awakening in the world after a century's absence. The quest takes her to the post-Cataclysmic Atlanteans that settled in Skartaris, revealing that Hephæstus was one of them, and "Atlan" is just what the Atlanteans call Poseidon.