Starcraft 2, like it's predecessor before it, has a very robust level editor that allows you, the DIY modder, to create nearly any scenario within the bounds of the Starcraft 2 engine that you could imagine. We have compiled videos on how to do all sorts of fun things with it, from making RPG's and Tower Defense games to just making you dream Starcraft 2 level. This video will teach you how to create a basic Warcraft 3 style hero using the Starcraft 2 patch 13 beta editor. He won't be quite as complete as the real Warcraft 3 heroes, but will have levels to gain, attributes to increase, and abilities to learn, so he's pretty close. With a little more work, you could definitely merge the RPG elements of Warcraft 3 with Starcraft 2's ubertight engine to create a really wonderful mod.
Download the map from the videos here.
Image showing how to do requirements for skipping ability levels here.