I see what you mean. The last "normal" companion was Martha. Even Donna was some kind of "most important woman in the universe" there at the end.
Except the whole point of Donna's arc was that even the most ordinary person could be the most important person in the universe. She was important because she just happened to be in the right place at the right time, and the importance of that coincidental presence at a moment vital to the history of the universe resonated back through time to bring her and the Doctor together.
And really, the same goes for Amy and Clara. Amy was an ordinary girl who was made special because of the influence of the crack in time, which was created by the explosion of the TARDIS as a result of the Silence's machinations to prevent the Doctor from bringing Gallifrey back at Trenzalore. Clara was an ordinary girl who was made retroactively special by her choice to step into the Doctor's timestream and save him from the Great Intelligence's attack on him at his tomb on Trenzalore. It goes all the way back to Rose, an ordinary shop girl who became a cosmic presence because she opened the heart of the TARDIS and absorbed the time vortex. And Martha, who became the most important person in the world in the Year that Never Was because she went around the world telling the story of the Doctor, enacting his plan.
So if anything, the recurring theme isn't that the companions are special, but that the
Doctor is special, and his companions are only made special as a consequence of his influence on their lives. This is particularly pronounced in Moffat's run, where every single story arc revolves around the Doctor himself, his enemies' attempts to destroy him, and his attempts to avert one ultimate fate or another. Even River Song was created to be a weapon against the Doctor. If anything, it'd be a refreshing change if we got a companion who actually
were special in their own right rather than merely as a side effect of the Doctor's battles.
EDIT: Heck, now that I think about it, I realize that Donna and Clara are basically the same character. The Doctor meets her briefly, then runs into her again later and takes her on as a companion. She becomes his best friend in a bossy and domineering sort of way. He wonders why their paths kept crossing and what's so special about her, but as far as he can determine, she's just an utterly ordinary person. Finally it turns out that the reason they're connected is because she happens to be there with him at a cataclysmic, season-finale moment in which she's transformed in a way that binds her to him retroactively, echoing back through time. The only difference is that Clara sticks around after this is revealed. Well, and also that there was flirtation between Clara and Eleven, but that was never on the table with Donna and Ten.