Provided I had the resources available, I would:
Create a world with varied cultures represented across national boundries. Certain areas/cities would be multi-cultural, but other areas deeper within the nation become mono-cultured. Those races who would be considered "strange" to that culture may have more difficult NPC interactions.
Create varied landscapes to explore. From mountains to deserts to jungles. Also I would create underground and underwater areas to explore and possibly a skyborne realm (where the player pans the camera and they are surrounded by clouds)
A massive beastiary. I would try to create unique takes on beings from classic myth and literature. This will include huge creatures like giants and dragons. Some few creatures would be large enough to climb upon and have an entire battle while climbing on them.
I would implement mounted combat....including aerial combat. I would want the character to be able to ride a pegasus or griffon or dragon or some such flying creature and be able to do battle among the clouds.
I would have naval travel and allow for the possibility of naval battles (see Assassin's Creed 4 for a ridiculously awesome example of what this would be like) complete with uncharted islands waiting to be discovered and explored.
Because I'm the lead designer on this game, I would have a very detailed martial arts culture in the game, both armed and unarmed styles would be represented.
I would have a complex, yet intuitive battlesystem that was varied enough to keep combat fresh and interesting. I would also implement a critical hit system that was dependent upon a combination of the attackers skill, what weapon they are using, what part of the opponents body the weapons struck and what armor they are wearing. Any body part hit with a critical stops functioning until it can be healed (through use of a skill or spell) so of course that means critical hits to the head are instant death and critical hits to the torso cause a significant disadvantage. Because of this, critical hits would be rare....but not too rare. Armor is important. Any location on the body not protected by armor is much easier to critically damage. Larger, tougher creatures such as giants and dragons will be very difficult to crit with mundane human-sized weapons (like being stuck with a needle or pocket knife) and of course certain creatures like slimes and certain types of undead will be largely immune to crits (example, zombies are much less susceptible to crits except to the head location)
I would have a natural world seeped in magic. This would make for all kinds of wonders found throughout the world such as areas where magical storms sweep through, draining everything of their excess magical energy (mages get all their magic points sucked dry!) or a desert with an oasis that holds a boulder of never-melting-ice (if you chip some of that ice off, it never melts and can be placed in a small body of liquid to freeze it solid...I would make that handy at some point during the game) or a waterfall that flows upward or an area where the clouds are as solid as soft earth. Magic would be common, but powerful magic would be rare. All characters would have some sort of magical ability from fighters (mystical martial arts stuff) to thieves (magically enhanced stealth abilities) all the way to full blow magic users.
I would have a complex narrative that was very important to the game. And things will happen with or without the characters involvement. For example, if the characters are told by the local king to deliver a token of peace to a neighboring kingdom where some hostilities are brewing, and the character party gets distracted by stuff in the world and deliver the message late, they will find that the hostile kingdom will have invaded their own kingdom and they are too late. Now when they return home, they return to a land occupied by enemy forces. However, if they were to deliver the message on time, the invasion does not happen and the home situation is different because of their actions. It would also be possible for the character party to return home just as the hostile forces are invading and become involved in the battle and possibly effect the outcome.
And for the love of the gods, I would NOT have it in First Person Perspective. I freakin hate that. I want to see my characters. And a party of characters, not some lone rambo who can take on the world all by his lonesome.