I think it would be interesting to have a world with only humans and one other sentient race. The humans would be dominant, but they would have to worry about the other race. Maybe it's orcs or beastmen. Even better, skaven would happily take over the sewers of human cities. Even just having elves would be cool.
Of course, there would still be monsters. Something like owlbears and griffons would still be around, but only as strong animals. Even some "smart" monsters could be made to have animal intelligence, especially ones like the minotaur. Or have intelligent minotaurs as the rulers of the beastmen.