The very first post is provied with the link. Just click on that link and you'll find out what you need to know.

I was also thrown by that, since her name wasn't mentioned in the trailer.

I watch the show for the characters I know and love, and there's still plenty that hasn't been explored about those characters. For example, if you read Eva's bio on CN's TDI site, it's promising and hints that the character could contribute more than has currently been given to her. She emigrated from Eastern Europe when she was 7, faced down the kids who tried to bully her when she first arrived in Canada, holds kickboxing records, works as a stuntwoman, hates cheaters more than anything, and according to her coach has real leadership skills hidden under her temper. But in 3 seasons (since we already know she's not competing in TDM) she's never gotten more than a few lines, and only her temper has received much focus. Only having 2 veterans return for the fourth season would pretty much guarantee I'd never get a chance to see her participate in an actual season, because you know it'd be the two faces they figure are best for grabbing ratings. We all have lots of favorites at this point, whom we want to see.

There's also a certain limit to how many teen stereotypes the creators have to choose from. All schools have their bullies, their nerds, their popular kids, etc. It's one thing to see them reimagined from one teen movie to the next, but when it comes to having all the old characters replaced with new characters inside a tv series where they have ongoing stories and relationships... it's not the same. There's that feeling of being ripped off that viewers who are invested in the established characters inevitably feel. I'd probably end up comparing whatever new party guy they came up with to Geoff, the new surfer to Bridget, the new nerd to Harold, etc. I'd rather they stopped at three Total Dramas than created another series with new characters, whom I would dislike from the beginning for displacing the characters I really want to see. It'd be like turning on 6teen to find nothing but new versions of Jonesy, Wyatt, Jen, Caitlin, Jude, and Nikki.

None of these characters has run out of story anyhow. The only way that the writers of the series could end up with that problem is by never developing the characters and only relying on one or two traits and gags forever. While TDI/A sometimes runs dangerously close to doing that, it seems to recognize the danger and push the characters before passing the point of no return. Cowardly, DJ stood up to Chef to get out of his alliance, Trent changed drastically and now has quite a lot of story potential without being focused strictly on Gwen, Geoff and Bridgette have thankfully stopped making out, Geoff's fame is going to his head so he's no longer the nice guy, LeShawna fell from grace by backstabbing her friends, etc... Lots of the characters show unexpected changes that shake things up already. Adding a couple new characters is okay, because that can also trigger changes in the established characters that allow us to explore other sides to them. With mostly all new characters, it might as well be a an entirely different series.