Ha, this is traditionally a Visual Arts forum question, but it's relevant here too.

I think you're getting a little ahead of yourself if you haven't programmed before. There's going to be a few Pongs and Tetrises before you can really consider making a 2D RPG of any scale. That being said, I would lean towards 2D since that's going to be easier to programmer art IMO rather than learning to sculpt, texture, and rig 3D models (although you might get a huge side benefit later on from learning that).

If I were you, I would keep a designer's notebook for your "Ultimate Project" but not really plan on implementing it as a whole until much later, like your 3rd year. If you're really passionate about this game, I would try to work on projects that prototype specific features, like a 2D tile game with only the most basic gameplay to get a feel for tilesets and maps, for example.

Basically, sit on your grandest ideas until you know you can implement basic ones. It's always much harder than it sounds to get even the simplest thing running, and running well.

EDIT: Grammar

I gotta agree with Prinz; prototyping small aspects of your game over time will be really beneficial to the development of you game, and ultimately, you as a programmer.

But now to the question you asked.

You might wanna check out a game called "Don't Starve", its a pretty epic game with 2d sprite work, a massive world to explore that used 3d for land sets, and is a bit of an action/survival game. I think this style of 2d/3d works beautifully and might be what you were thinking? Worse case, taking a look at this game can give you some ideas, right?

Now if you start leaning toward 3d, (which personally takes me less time than hand animating sprites) there are some things I would suggest.

In your post, you said 3d ages poorly. Well, I think it really depends on certain factors. Take the 3d models and style from Metal Gear Solid on the PS1. The characters are well done and stylized. They look better than some other models of the time (and in my opinion, better than some stuff today) because they have an established style. Just the same as the characters in Legend of Zelda all adhere to a style, thus creating a aesthetic that is pleasing to the viewers eye. Then look at games like Fall-Out 1 and compare it to Fall-Out 3. Fall-Out 1 has a consistent art style; not the fanciest or the most detailed, but its easy to get enveloped into that world where as the art for Fall-Out 3 is trying to be realistic, in your face, and detailed. When you have high detail on a characters armor, but the low detail on the poster their standing next to, it starts to take you out of the world. To me, the immersion was broken in Fall-Out 3 because of the whole uncanny valley thing.

So yeah. my suggestion would be to try to make all the art consistent through out the entire game and also don't strive for super-realistic shtuff and you'll do fine with whichever path you chose. It'll take some years to learn, but you knew that already.

Best of luck!

-Dave