So, DC seems to making a genuine effort to increase Nubia's presence and make better use of her. I think most would agree that's a good thing. I think she can be a welcome presence in Diana's life.
Problem is no one seems too sure what exactly her origin and backstory is or should be. Admittedly, this relates to DC as a whole, as continuity is all over the place across the board...up to and including Wonder Woman herself (how old is she now? was she in World War II? still Zeus' daughter? was she an only child?)

Anyway--staying on Wonder Woman--although many specific details are subject to change and fans have their own preferences, there are always crucial beats that almost always stay the same no matter the continuity or interpretation.
In Diana's case, she's never just an Amazon, there's always something special about her--whether she's the first/only child of Themyscira, or the daughter of a god, or blessed by the gods. Whether it's during a World War or modern day, her mission is always heralded by Steve Trevor crashing onto Themyscira. Whether she enters the Contest or not, she always leaves against her mother's wishes.

So, when it comes to Nubia (or Nu'Bia as she was called Post-Crisis) we've seen a few variations on what her deal is. And, like Diana, I expect we all have our own preferences for her origin and backstory, but what are the crucial beats that define her? What, no matter how she's re-imagined, are the things that Nubia must have to be who she is?

- Particular kinship/link to Diana
Arguably her most defining element. The most famous take on this being she's Diana's literal twin sister. Sometimes she's Diana's twin in a more figurative or metaphorical sense. The only exception I think is Post-Crisis, but even then, they were linked as Amazon Champion--Diana being the latest, Nubia the very first (and greatest?). Whatever the case, Nubia is never just another Amazon. There's always something that links her to Diana specifically.

- Diana's equal?
Tying into their kinship, whether it's because they're twins or she's blessed by her own gods--or she's just that badass--Nubia is usually presented as Diana's physical equal or at least on par with her.

- Separation & Reconciliation
A recurring element is Nubia and Diana are always separated for most of their lives. Pre-Crisis, she was kidnapped by Ares. Post-Crisis, she goes into Doom's Doorway for a thousand years. Maybe she just left Themyscira altogether? Whatever the case, it leads to her and Diana reuniting and, more importantly, reconciling after some initial antagonism.

- Harsher personality?
Whatever separated Nubia from Diana usually plays into her having a harsher or more aggressive personality. Maybe? Whether it was being raised by Ares or spending so long behind Doom's Doorway, Nubia contrasts Diana's idealism and compassion with a more pragmatic and serious approach.

I haven't been following Nubia & the Amazons (yet) so I don't know how much here--if anything--still applies.
Agree? Disagree? Am I missing something?