I'm not sure what other people have said but these are my suggestions:
Have a safe word... make it obscure like "Tunafish" so if anything goes wrong and she thinks its too much she, or you, can stop it immediatley. The word "No" and "stop" should mean nothing as she might get off on begging you to stop raping her but really, she wants you to carry on. The safe word allows this to happen with you being aware and happy she is safe and is enjoying herself even if she is begging other wise.
Set the scene... for a rape fantasy - you could set up an abduction. Set the scene by creating a night she is walking alone down a familular street and you bundle her into the car (in a set destination so she knows its you) and then do what you would like to her either in the car or at a destination. As you both become more comfortable with this - you could leave her a note saying that one night this week you'll take her and then go off your own steam of when and how - meaning she has less control which might fulfill her fantasy more. Talk about it in advance.
Get ideas... As it sounds like you are a gentle lover - maybe have a look at some porno's to get ideas on how to be a little more forceful. Don't do anything you are not comfortable with but start small like pulling her hair, dirty talk and tieing her up before you develop onto bigger things like whipping (if shes into it) , breathe control (if she's into it) and set fantasies. Be forceful with holding her down with your hands, if you are hurting her she can always say "tunafish" which means you can let off a bit... DONT STOP completely as it can ruin things but just be more careful before you work your way up to being more forceful. Ideas for being rough would just be to chuck her around in lots of different positions, hard fucking, throat fucking (if shes into it) degrading her by cumming on her face etc etc
Spider Chart... both you and your girlfriend could spider chart three things....
1. "Every Sexual activity you've heard about"
2. "every sexual activity you've done"
3. "every sexual activity you'd like to do"....
be totally open and put down everything you can think of or has tickled your sexual interest in the last one because it'll give you ideas what she'd like to do without her having to vocalize them - this is a massive bonus if she wants you to take control as it'll take away the knowlege you've had an indepth convo about it which makes some people loose the interest.
Talk about everything but the key is confidence. If you have confidence - it'll make her fantasy so much more realistic!!
You're amazing for wanting to do these things for her - but make sure you are happy too!!
C x