An RPG where you are a 'demon-hunter' (NO NOT LIKE THE BLOODY WITCHER!!) which merges RPG elements with Survival-Horror elements and you do not have just to attack the demon/ghost/vampire/etc.. with your +2 sword of demise but you actually have to solve puzzles, use strategy etc. Bashing the monster with the sword MIGHT be an option, but would be little rewarded.
The XP system rewards you after defeating an enemy (ok nothing new here) but not only the result is important. XP will be rewarded on HOW you killed the monster.
A simplistic and dumb example would be... let's say... 'kill a vampire'. You can use your silver sword to kill him charging him head on. It will be hard and since it's a dumb way to act (they are mighty foes after all) if you survive you get little XP. Another way would be coating an arrow with garlic and use stealth and try to hit him in the heart. Or Magic. No not 2d8 damage fireball. But maybe some kind of magic ritual where you perform puzzles to 'perform' and defeat the vampire after luring him in the magical pentagram.
If there is ?bash your enemy in the head? combat in the game, it should be against weak enemies, mostly humans (for example the people enslaved by the hypnotic power of the vampire mentioned above). This enemies of course yield no XP (or maybe just the first one of it?s kind you kill of if you use some fun or smart technique to kill one of them)
Of course to combine this gameplay with sand-box gameplay might be difficult (since one might primarily use a linear gameplay) but if the team of creators is inventive enough it could very well combine the 'oblivion-like' experience (or maybe the 3rd person perspective might be better in terms of gameplay) filled with freedom to run and climb everywhere and the 'survival-horror' experience where you are a weak human against terrifying foes where your mightiest weapon is your smarts. The foes are not cellar rats you just stab, but mighty supernatural foes, forcing you to use your brains more than your brawn.
Then of course there will be skills, research, exploration and handy 'objects, armor and weapons' that aid you in your quest.
Another part of the game would be a ?detective? part (which is usually a part of the survival-horror theme). You do not just explore a location for a chest containing the Holy Grail and the +69 Staff of Penetration (thank you Yahtzee!) but you actually research it, maybe learn its history, find how you can use the environment to your advantage. And of course the monster will not just present themselves to you. You will have to find them, dig deep to find out what is that causes all the poor virgin maidens to die in that village nearby that spooky castle (still making the obvious and unimaginative vampire example)
As a final remark I want to say that the RPG should have a kick ass plot and possibly be open ended (avoiding extreme black and white ends, or maybe include them along with more shades of gray ends). Again to the vampire example: one could choose to kill the Vamp, maybe accept his proposal to let him live in exchange for a reward (and here it?s very black and white), but one could then still kill the enemy (or let him live) but then, for example, find the source of its powers (the magical vampire tooth or whatever) and use/abuse it, either to help people, to take revenge on that prick that made fun of you in school, make money (or other not pure evil but still selfish reasons), to make yourself stronger? of course such power will come at a dire price, etc.
It is not an easy task and it is often replaced by either no choice at all or extremely black and white morality.
And is some guy actually makes this game, I called it first and I want some credit! Money is also good

LOL
Oh yeah... this is not really science-fiction setting, but could be. It does not have to be necessarely set in the middle ages, but in the future.