Huge project paid off for Notch.
I think this game could be successful. Lots of people would like to make games, but most cannot because they underestimate what programming involves. The next best thing for these people is a game with a comprehensive editor. I think that's why Minecraft is successful -- it's essentially an editor for people to make their own game world. What it lacks is deep adventure, and that's what my game idea has tonnes of. So I think it could be successful.
Another example is warcraft 3 -- the comprehensive world-editor invited a modding community to grow and develop custom games, and that gave birth to dota.
I think the hardest thing with the programming is to maintain enough versatility that it's possible for users to design their own content. The hardest part of
that is designing the classes and structs for everything so that they fit together harmoniously.
For example, an item could do a myriad different things: It could be consumable. It could be a weapon. It could be wearable. It could produce light. It could produce particle effects, etc.
If items were hard coded, it would be easy to do all this, but when we make items editable, the item class has to be versatile enough to allow for all kinds of obscure permutations.
Once that has been done, the editor itself should not be too difficult, and I enjoy making editors. Once I have the game engine + editor, then I can use it to design the default dungeon without doing any more coding. (and I can debug the thing at the same time)
Once a modding community gets up and running, users will no doubt start creating addons, such as programs that produce procedurally generated dungeons, gauntlet runs, and things like that.
I get the feeling your RPG Maker like idea will take much longer to code than Minecraft. Also, take into consideration that Minecraft took at least several years to code(I think?).
And, you don't offer anything relatively new like Minecraft did. Even if you pass the first two or three years where you don't have something people would want to use for their own games, it's not really certain you can build a community
But, that's just my opinion, from my perspective. I am not sure I got it right.
However, presonally, I would much rather play a horror themed bear game in a couple of weeks, than wait 2 years for yet another generic RPG game maker.
My 2 cents.