Things to watch out for (From my own experience)
1. Artwork: The artwork for an RPG can be intense. You may not think so at first. Most of the time you can get away with generic freebies for terrain tiles but all the little things take a ton of time. If you plan to download items, weapons, enemies, decor tiles from somewhere make sure that you find enough in the same scale (i.e. so that chairs, mugs, sprites, fit well together).
Rather than drawing transition tiles seperately, look at creating a tile-system that incorperates a fringing layer (if you need me to clarify just post).
2. Get an understanding on how to write a scripting interpreter: Believe me things (triggers,events,items,npc's get crazy right from the beginning). You need to have a way to place these things without having to re-type hardcode. Your engine will need to read these from a file.
3. Design document: You need to hammer out pretty much everything. My biggest problem was sticking to my decisions. It's hard not to look at the latest released RPG and say, "darn, we should do it this way". Changes like this will slow down your progress, and take away your own enthusiam over your project.
4. Stay on course: I spent a lot of time running around in different directions. At one point, instead of coding, I started learning Lightwave 3D because I wanted to create cut-scenes. It cost me a lot of time, and I realized that although I am good at drawing, it does not mean that I am good at 3D-modeling.
5. Do what your good at: RPG's are hard enough for people who know the language and have worked on many smaller projects. Do not start an RPG game thinking that you will learn as you go. It takes too much time that way. And by the time you learn things, you start realizing the code you wrote 6 months ago is all wrong. It can be disaterous... plus it will take so much time to complete the game that you and anyone else working with you will lose interest in the project. If you plan on learning create smaller parts by yourself before putting together a team. Understanding, that most of your early code will never make it into the actual game.
6. Keep your ideas realistic. Coming up with great Ideas isn't the hard part. But it can easily overwhelm you to combine all these features into an actual game.
Some other things:
7. Read everything about RPG design you can get your hands on. Don't write any code until you are 99% sure of what you plan to accomplish. Set realistic deadlines. And stick to them at all cost. If you have a group of 5 helping, plan on being the only one working on it after a month or two. People tend to fade out. Especially if they aren't paid professionals. Design the game from the beginning with the idea that you will be the only one working on it.
Dump your girlfriend.
Quit your job.
Tell your parents you will see them in two years.
Buy lots of Mountain Dew or JOLT!
Get used to eating cold pizza (from a week ago).
Plan on not having a tan.
Don't read about any new RPG games coming out!!!
Don't buy any new games!!!
Don't play any games once you have begun!!!
All of this will give you at least a 9% chance of completing the project.