There's nothing you can do to really make your playerbase like the Rep side more than the Imperial side. It's a matter of what the fan already likes. Me personally, I prefer Republic because I enjoy the story and what-not more along with some of the abilities.
Unless they say, oh, we've added Yoda's race or, you can now play as Luke/Darth Vader, there's really nothing they can do. It's all a matter of what most players prefer honestly.