if there's more rumbelle in the finale, than I would say that's what Robin Hood would be tied too. Not anything to do with Tamara and what she's doing.
Thing is though, Rumple appeared to be done with Robin after the events of Lacey. Belle convinced him to let them go, and he did, so while it may be in the back of his mind that he wants that wand back some day, as long as they're exploring Rumbelle in FTL, I can't see Rumple actually initiating more of that storyline by going after Robin again.
So if Robin's gonna be involved in the final eps, it will have to elaborate more on his own story. And given that Robin Hood is so iconic, I'm sure they've come up with a specific story for him. Given that they made the departure from the traditional Robin Hood story and showed Marian as being pregnant, they must be planning on elaborating more on that. If they just needed some pregnant chick so that Rumple would have an attack of the guilts and not leave the unborn child fatherless, they could have made the pregnant chick anyone. But they chose to put Maid Marian specifically in that position. There must be a reason.
Meanwhile, we have Tamara, a new character that they're going to elaborate on more in the final few episodes. If they're gonna show her history and motivations, and Robin's history too, then it would make sense if they're intertwined, otherwise they'd have double the amount of separate stories to tell in not many episodes.
Also, if Tamara's deal is that she's trying to find her long lost father, Robin Hood, and that's why she's been collecting magic, so she can find a way back to FTL where she believes her father still is, that potentially explains how she met up with Greg, who has been searching for his own father in some mystery, magical town.
That's what keeps me from ruling out Tamara being the child of Robin and Marian, even if I'm not 100% convinced of it just yet. I can just see lots of potential for them to tie things together really nicely in the space of just a few episodes if it's the case.