Yup, working out the math and the system is really the clincher here. You need to make sure to have a system that's fair; and if you do get into status ailments, elemental strengths and weaknesses, and weapon qualities; it can get pretty complex pretty fast. A list of what you'd need:
- Method for deriving damage from attack and defense values (possibly with randomization involved)
- Determining how evasion and agility play into defense.
- Method for deriving damage from Magic attack and defence values.
- Luck should be a very sublte effect based on randomization. It's likely that you could somehow use dice for this, but now we're almost getting into a computer-programmed solution.
- Effect of different spells on M. Atk and M. Def. Are some spells more potent than others? If so, you'd need a list of available spells with their attack values.
- Effect of different weapons on attack and defense. As with spells, some weapons may be more potent than others, and would need to be planned out.
- Status effects like poison; what are they, what do they do, and how do you determine the outcomes of those effects, stats-wise?
Personally, I think it's a really good idea. But if you're starting from scratch, you have a LOT to think about. True, you can dumb down/mitigate a lot of these questions by keeping it simple. But there's still work to do.
Suggestion #1: Take this thread over to the Worldbuilding forum, and get help on working out the exact details, recording the ones you want to keep as you go. Let everyone dig in and help a little.
Suggestion #2: Use something pre-packaged that deals with most/all of these issues. For instance, use the d20 system (Dungeons and Dragons et. al (even though that'd be a little cumbersome to make and play characters, and everyone would need to go get the SRD at least)).
Suggestion #3: Write a computer program that handles the majority of the calculations for you. I can tell you from experience that programming is a very fun activity, and it doesn't hurt anyone to learn.
I think I'd recommend Python as the language of choice for something like this because of its ease-of-use, interpreted nature, and simplicity. But I don't have much experience with that particular language, so not sure.
Rant over. You may now ponder your existence.