Setting: Arcanum: Of Steamworks and Magick Obscura
Gameplay: Fallout: New Vegas
A first person Arcanum RPG would be fucking amazing. I want to explore that world in first person. Dialogue and quest design should be the primary focus, which Arcanum already did well. Toss the damn quest markers. Borrow some aspects of the crafting and exploration from newer Fallout games.
Unleash a mixture of steel, magic and technology in battles. Suffering magical or technological failures from them disrupting each other in real-time FPS combat could be interesting. Stats and skills need to matter both in combat and out. It would be interesting to see your character build affect the FPS combat in ways other than damage done or mysteriously "missing" (Morrowind.), perhaps aiming hindrances/assistance, recoil adjustment, rate of fire, etc. There is a lot of opportunity left on the table for merging the RPG with FPS combat to put the player into the shoes of the character better than what we have now, much more than simply doing more damage with a better build.
I would gladly take a proper Arcanum game/remake with competent turn based combat any day, but it is a world that could be really compelling to explore in first person.
For example, there you are, enjoying your zeppelin flight in whatever part of the ship your background supports when you hear the racket of crude engines. Maybe you see them clearly from the deck with the well-dressed elite or maybe you just catch a glimpse of them through a mildewy porthole in the cargo hold, but either way, the fighters make short work of your blimp. The ship crashes all around you, but somehow you survive. Twisted metal, splintered wood and ravaged bodies are strewn across the dark landscape, pockets of burning rubble illuminate miles of wilderness in every direction. You quickly gather your senses and pick about your surroundings. Maybe you snuck a shoddy pistol aboard, or maybe you scavenge a steak knife from the crash. Or perhaps you feel confident in your ability to magically inflict harm, should the need arise. But before long, you spot the shimmering eyes of carnivorous wildlife, pressing in on the calamity...