Incredibly hard it may be but depending on what you want it can be done. Normally an RPG becomes another RPG but you can reasonably add overtones to a game: give a racing game and RPG slant or beef up the RPG aspects of an action game. I am presently off translations for a little while and focusing on stuff like this (although nothing quite as in depth as it appears you want to do).
Such a hack is called a total conversion and there are precious few (
http://www.romhacking.net/hacks/143/ ) of them outside of stuff with official modding tools (
http://www.moddb.com/mods?filter=t&=Se...ef&type=def ), the next step "down" is a complete hack which normally means a sort of game 1.5/fan sequel or a hard mode hack
http://www.romhacking.net/?genre=&plat...le=&author=
Easy stuff: text replacement, sprite replacement. (they may be relatively easy but done correctly they are very effective: see retranslations)
Harder: stats altering, weapon replacement, map/objective tweaking.
Harder still: adding entirely new stuff (changing a pokemon is easy but creating a 152nd one is quite hard most of the time for example, even more so as most games use the full multiple of 2 for things)
Harder still: redoing AI and simple game mechanics (making two attacks where there was one).
Hardest: complete overhaul: adding magic to an RPG where there was none before, adding team mechanics. Turning a racing game into an RPG.
This is just a rough guide: occasionally you might find a nice XML table or something for weapon stats while text is a compressed, non-relative set of files with loads of Kanji. 3d graphics are generally a bit harder than 2d but if you have the format 3d is usually just simple geometry while 2d sprites and the like can be a puzzle (especially with repeated sprites in backgrounds).
Post back with what you want to do, if this is you first time at all this I suggest that you do a basic story/text hack and keep working on stuff releasing new versions.
edit: a good example of that would be powder (warning big page)
http://www.zincland.com/powder/index.php?p...mp;maxentries=0 . It has been playable since day 1 pretty much but it keeps getting better all the time.
