Try Phantasy Star Online Episodes I and II. It's really had to find, but if you don't find it, there's always eBay. I looked for it about 2 or 3 weeks after its release and found the last one in my entire MALL (yes, I went to every store that sold video games and I found the last copy at the last store I'd visit, Target). When you say you like the Phantasy Star series, I assume you mean 1-4. Nonetheless, give this a try. I love it, and it's great to play a little while, then put it away to beat another game, then when you've beaten that game, play more PSO. The online function really makes it great to play forever.
Second I'd suggest Skies of Arcadia. I personally have not played it (I'm getting it for my birthday in one month), but both my best friend and my cousin says that it's great.
The other RPGs for GameCube that I know of are Evolution Worlds, Boulder's Gate, and Rune (known as Lost Kingdom in the US, I think). I wouldn't really recommend any of them.