It would be sort of interesting.... I remember back when you created a character in RS classic and you could choose your character type during the creation process. Your choices were something like: wizard, warrior, ranger, and depending on what you chose you started out with certain combat items to match.
I think having different races & character classes would be kind of cool, and would probably make a big difference in how the game progresses, and the types of things and abilities your character is able to do etc, but I also think RS is the way it is for a specific reason.
The one nice thing about all starting out equally, as human characters, is that nobody has any sort of advantage over another person when they begin the game. We all are the same, have equal abilities, and progress at the same rates as each other through the game.