Donna was in development for a DCAU-based Teen Titans team, but I don't recall it being explicitly stated she was a part of the initial roster for the '03 series.
Paul Dini did state at one point that
I found the quote on another site but the link to the interview is dead now. Probably could find a source somewhere on the internet, but this had to be before the JL cartoon. But, based on that if Donna wasn't intended to be a lead they weren't having it, I assume? But, it may be more layered than that because she couldn't be used in season 1 of YJ and I believe Greg Weisman stated she would've been on the original roster.There’s kind of a licensing problem: if we wanted to do Wonder Woman as a series, we could do that, [but] if it was a guest-shot, it was a little more problematic. I don’t really understand it—it just turned out to be easier all the way around [to use Big Barda in the Batman Beyond episode 'The Call'...] we love Wonder Woman—Bruce [Timm] did that great design of her, which is now a maquette at the Warner Bros. store. At some point, we’ll do Wonder Woman…we just need to fight that battle when we get to it...
Donna did make a quick cameo in like the very last episode of Teen Titans and appeared in a couple of issues of the tie-in comic, so I think there were plans to have her at some point. However, it's possible they didn't intend for her to be a part of the original roster, and thus that caused Donna to be embargoed from the show.
Although, now I'm curious if she would've been on the Titans East team if they could've used her.