True, she was not like Rose or Donna or Amy where they became important parts of the stories as well as part of the Doctor's life. After all, he fell in love with Rose and Amy was a strange best friend/mother-in-law combination. Donna's granddad was an important part of the Doctor's life in the end.
I still suspect that Clara may well turn out to have a connection to nostalgia of the classic era. Spitballing -She's Susan
She's Susan's daughter.
She's a new Guardian. (Recall the Black and White Guardians from the Key To Time saga)
She's The Rani.
She's The Doctor's other daughter.
She's Susan's grandmother.
She's the beginning or re-beginning of the Time Lords.
She's the Key To Time.
She could also bea Jagaroth
for all we know.
Actually, what do we know?
There's the 'girl in a shop' that gave Clara the Doctor's phone number.
Clara's last name is Oswald - Dalek Clara's her shipmate was called Harvey - Doctor who first aired the day after JFK's assasination.
The Doctor has met her in four time periods where she is linked beyond appearance and characteristics...
Future: Likes souffles. Says 'Run you clever boy and remember.'
Past: Like souffles. Takes care of kids. Says 'Run you clever boy and remember"
Present: Takes care of kids, greatest childhood fear was getting lost. Password is "RYCBAR"
A few years ago: often loses things