I think a Cartoony Story Book look would be a great idea. I think you're kind of using that as an excuse for your work rather than really playing it up, personally. That being said, especially if you're starting, I can't blame you.
My main recommendation is to take a story book that you like the art from, and use that as a reference. I'm looking up a few of them online, and some of them have some nice art.
If you're going to make them flat color, then I'd give them solid outlines. One thing I might suggest is maybe making it look like stage props, it kind of gives off that look at the moment, and maybe I'd lean towards that.
I did something like that to test. I could have put more detail and stuff, but it's not for my game so...
Well, in any case, I'd take a look at some childrens books and see if you can not only make them cartoony, but make them very imaginative and fantasy like too.
Off topic, but I was watching the 'Old man and the Sea' on youtube, beautiful stuff. I'd like to have a look like that for my game.