For example, much of the DnD kerfluffle is over the open rulebook and monster manual. If you make a game that has a Blue Dragon and operates totally differently from the "Adult Blue Dragon" that's in DnD you're probably OK.

But if you make a game that has an "Blue Dragon" with an Amour Points of 19, a Health of 255, a Speed of 40, but 80 while flying, and a Str 25, Dex 10, Con 23, Int 16, Wis 15, Cha 19...then you're a lot more likely to get into trouble.

So you make make a knock-off DnD that has pretty much the same _mechanics_ its true that you're probably fine. But once you start borrowing all the of stats of things from another work that fit into those same mechanics... well, I Am Not A Lawyer, but that's definitely the kind of thing that you'd want to start running past a lawyer.