Never seen Riku with Yellow eyes, Yellow eyes have only been demonstrated with people deeply linked to the darkness. So Terra having them implied he was going way down.
Then, well, Roxas is something much more convoluted than "Sora's nobody", when you think about it.
Reading the other person playing with the name Xehanort, I found that funny :
If you form "Earth" with the name, the only letters left are X N O, which could be "nox"
nox in latin : nox noctis f. [night]; meton. , [sleep, darkness, gloom, death]. Abl. form as adv. noctu, [by night].
Since Terra is latin for earth, it's kinda funny. ^_^