It does appear so at times, that Bethesda skip out or cut very interesting content. Regarding Masada, Indeed, it would be more of a spiritual inspiration, perhaps solely for the aforementioned grand keep. And seeing as how Orsinium is supposedly in the Dragontail Mountains, a desert plateau wouldn't do at all. Though, a large walled fortress located on an elevated plateau in the mountains, overlooking rest of the city could work, in my opinion. The new architectural style introduced in the Orsinium DLC of ESO I think would lend itself well to the rocky terrain, relying more on hewn stone instead of palisade walls. I like the idea of cliffside redoubts, as the city itself and a number of associated locations would be the main setting of the DLC (a faction-based DLC, like Tribunal or Dawnguard), the setting should be interesting to explore.
As for the Markarth idea, I like it. Markarth under siege would work, and it could even factor into the main quest of the DLC, dealing with Orsinium's defense and internal struggles between the more traditional Malacath-worshipers and the Trinimac worshiping and possibly Thalmor-backed Vosh Rakh. The Forsworn could be a suitable antagonistic force, besieging Markarth and assailing the Valley of Scars, providing a suitable test of Orsinium's defenses. The DLC could also re-introduce Giants, as they would likely be absent from Hammerfell in favor of more widespread potential enemies, such as Minotaurs in Eastern Hammerfell or the many beasts that prowl the Alik'r Desert, such as Dunerippers, Giant Scorpions, Assassins Beetles, Harpies along the northern coast, and perhaps a reimagined enemy from Shadowkey - the Spikeworm.
In any case, a somewhat reimagined Markarth could work, as they could recycle Dwarven architecture to construct at least part of the city. Perhaps in the time since the events of Skyrim, the Forsworn have retaken Markarth and reformed their independent kingdom, striking out at the fledgeling Orcish kingdom under the influence of Boethiah. A small series of quests would revolve around sneaking into Forsworn-occupied Markarth and sabotaging its defenses and weaken the Forsworn, potentially storming Understone Keep and eliminating the Despot of Markarth, leading to a Nordic victory and an end to the threat posed to Orsinium. All the while, tensions flare between Orsinium's two main factions, forcing the player character to support one or the other, to prevent further any division that could endanger the kingdom of the Orcs.