With all of the room on a cart, im guessing that it would be very possibly to fit something much more vast game-wise than say Mario Tennis. Something more in-depth like an RPG.

definitely. maybe we’ll see one one day. with the flashboy and vbjaengine being developed at the moment, there will be powerful tools soon, which will hopefully revive and boost the vb homebrew scene. with the flashboy finally everybody will be able to execute code on the vb very easily and quickly, and jorge’s engine will be very helpful to develop complex games. it is aan excellent piece of software i must say! can’t wait until it gets released at the end of the year.

rather than start a new thread, ill ask my ? in this one: is it possible,with the new VB dev. tool, to take an existing game (ex Mario Tennis) and implement/add a code or function to the game to allow 2 player ?

nope. you can’t just add stuff to existing game, except you break into the nintendo hq and steal the source code and the tools to compile it. 😛
you have the wrong idea of that “vb dev tool”, the vbjaengine is basically a library of useful functions which ease development for the virtual boy.