My dream RPG would have every single building be enterable, puddles to splash when I run through them, smaller objects that actually move when I bump into them, and the clothes on my character to react to the wind and the rain whenever possible. It's the small things that really makes a difference to me, the things that really immerse me and sometimes make me think, "wow, they really didn't have to do that but I sure as hell appreciate they put it in the game." As for combat, I want my bullets not to just leave a red mark on my enemies, I want them to react to the hit if they are vulnerable in that area, I want to see them crawl when I've taken out their knees, I want to see them scared or panicked when they're outgunned, and I want to see them using actual tactics instead of just charging straight in front of my guns. Generally, I want the AI to be really smart, and not just in combat either, but staying on the topic of combat, I want the guns to sound realistic. Sound can make a huge difference in terms of immersion, especially in situations where you can hear a sniper bullet cracking against concrete or shrapnel from a grenade hitting close to your position. Also another important thing to me, melee. In some games it's really satisfying, and in others I'm just left thinking, "I don't know if I hit him or if my weapon was just phasing through him." When I hit a guy in the face, I wanna see blood, bruises, dents or some other kind of damage.
Moving on, I could live without most of the things from the combat, in fact I could live without it entirely. The one thing that matters the most to me is freedom to do and say what you want. I don't like it where I'm pushed into a position where the options given are none of the ones I'd ever do. The most important part of a game for me is the characters, and the interactions you have with them as a player. While I'm on the subject of interaction, I'd like the random chatter you may overhear in the game not to be just the same line repeated over and over again. It gets really annoying, instead I'd like them to do a lot of voice acting for the random characters to the point where you could do a second play through and hear things you've never heard the people talking about before. This was a pretty big problem for me in Dragon's Dogma, I absolutely loved the game but the fact that the pawns didn't have much to say annoyed me, especially seeing as they're with you through the entire game.
That's pretty much it for me. There are things I can live without, I don't even really need pretty graphics to find a game immersive so long as there is a lot of really cool and varied content to keep me happy.