So, there has been interesting rumblings throughout the internet about Sabine, and I have to admit - it seems really likely!
Background:
It all starts with Disney's cancellation of the Clone Wars when they took over. Dave Filloni and crew had scripts, story boards, and pre-vis work on Clone Wars episodes and story arcs all the way through seasons 6, 7 and 8 (when the show was originally slated to end) detailing the arcs they wanted to include. Since season 6 was most of the way through production, they finished those episodes and they now appear on Netflix as "the Missing Tales" along with seasons 1-5 of the Clone Wars.
Season 7 was almost fully scripted and even had some pre-vis animation done (you can see some of it here). They took these scripts and handed them over to other parts of the Mouse House to be developed in different ways. A story arc involving Quilan Vos and Assaj Ventress is being converted into a novel coming out called "Dark Disciple," while the final story arc for Maul was converted and published as the Dark Horse comic series "Sons of Dathomir." This comic series is where it gets juicy.
The Reveal:
In this series, we see Maul and his Shadow Syndicate waging a hidden war on Sidious and Tyranus, enabling Maul to get revenge on his former master and prove himself superior to his "replacement." Maul's chief lieutenants in this endeavor are a pair of Mandalorian Supercommandos; Commander Saxon (Maul's right hand mando) and Rook Kast. It's Rook Kast in this one that is the interesting bit - not just because of the fan-girl geek out opportunity involving the reference to Jodo Kast either. Rook Kast is rumored to be Sabine Wren's mom!
The Evidence:
A few things make this seem really plausible!
People's Exhibit #1: Here's a picture of Rook Kast's helmet & Sabine's next to each other;

Obviously not the same helmet itself, but if you'll note the similarity in the artwork on it. Dave Filloni has twice mentioned that her helmet was a gift and the only part of her style she never repainted (once in answering a question on his twitter feed, and again in one of the Disney XD vidoes hosted on the Star Wars Rebels facebook page). This makes it reasonable that if Sabine's mother were to have given her a helmet that bore the same design that her own did, it fits that the symbol could directly tie the two characters!
People's Exhibit #2: Birds. Both Rook and Wren are birds, as well the imagery on their helmets has a very avian look to it. Even her choice symbol is that of the Wren, showing that the tie to birds is definitely important. Being part of the underworld like both Rook and Sabine are, it's reasonable to assume that they would adopt names as alias' to better cover their tracks, meaning that despite the lack of common last names, they may be related if Sabine took a name to reflect her interest in avian wildlife.
People's Exhibit #3: Mostly a counter-point, in that it is often said that Sabine could be Bo-Katan's daughter (which even I subscribed to until Dave Filloni squashed it). He has on multiple occasions stated that Sabine isn't related to the Kryze's - which rules out both Satine and Bo-Katan.
What do all of you think?
Edited by Kyla