Whoever this woman is, she will be able to "crack" open the "scar" on his chest, which we now know more than likely links Stefan with the Phoenix stone (and the possible confirmation that he may die and be brought back--due to killing Julian; but, I'm getting ahead of myself...). We don't know all of the information about the stone, but it obviously can do one thing very well: mediating the in-between state of death and life, right after you die more than likely. It has the interesting quality of making the spirit that has inhabited the body, use that bodies mechanics to live upon.
The man in Oscar had to become a vampire again, IN an already vampiric body. That is a potential cure and a HUGE doorway for other things to eventually happen. But, when they brought back Julian he had that little "mark" left upon him. The mark could be caused by several things: part of the ritual to get the soul to enter the Phoenix stone, something the stone itself creates to connect the dead body to the spirit--much like the Anchor for "The Other-Side, and it could be a mark leftover from the resurrection process using the stone--a type of "brand" to let others know that you've been in the stone or have something to do with it (as it may have been a "prison cell"; those let out for time served had the mark placed on them potentially so the witches that used the stone could manipulate the "branded"--or use it to manipulate "probation" for the incarcerated, etc...; there are other possibilities to, many becoming far darker versions of what I just gave you; it may also be possible that the mark we see on Stefan in the future is actually the one that is on Julian right now).
BUT, here is the real question, besides those above... Who do you think our mysterious Arrow woman is? I'm going for Valerie or possibly Lily; but, out of left-field I'm hoping for Katherine. Katherine doesn't have to be in Elena's body (though it would feel just...wrong). I'm not sure who to pick right now, but Lily seems possible.