One thing is... how to distribute enemies/XP/treasure/quests.
Provide the player with positive reinforcement for "doing things". Do it too infrequently, and they get bored. Do it too often, and it becomes pointless. The balance also depends on the specific mechanics. Equipment drops in Diablo and finding new materia in FF7 are on totally different scales, but they both work. Why is that?
I'm being flippant, but pose a vague problem, and get a vague solution. The devil is in the details.
I am sure you are pro at tweaking stats.
I can't speak for anyone else here, but I'm not. I can tell you when a game gets it right, but that's about it.
I would think there are textbooks on game design that would give you a decent introduction to that sort of thing. Asking random people on the internet for input wouldn't be my first choice.
I used to play a lot of RPGs but in a recent decade I didn't play many games at all.
You wanna write a good book, you gotta read stuff. You wanna paint a good picture, you gotta look at stuff. If you want to make a good game, you should be sampling the state of the art. From the style of your graphics, you should probably be looking at a lot of indie games. They can do some extraordinary things with minimal resources, but it's a good bet they're paying attention to their competition.
So basically I need your tips and help on how to develop this RPG game.
Bluntly, that's not how the internet works. If you ask specific
questions, or want feedback on an alpha release, you might get some useful comments. The internet loves to share its opinion. The internet will help out when it can. But the internet will not work for free.
My general advice: in order for a game to be successful, it should do at least one thing really well, and do nothing particularly badly. Figure out what you can do well, and focus on that.