Here's my thoughts on Nakomi.

Tigraine? I don't think so. Avi spent a very long time in the Royal Palace. We're told about the paintings of the Royal Family, and Tigraine's in particular, hanging in the Palace. For me it's too far of a stretch to think that as observant as Avi is she would not outright recognize her. Or at the very least be thinking this woman looks very familiar. It's possible she changed her appearance, but I'm not sure how we would get any clues that it is her this way (which is alluded to). The Hero of the Horn theory would have her looking like her last incarnation from what we know of those mechanics.

The Creator or their avatar? As others have mentioned RJ explicitly states The Creator does not take an active role. So for me this rules out this possibility. Though it does show a balance to the Dark One and his own Avatar. Unless someone with access to RJ's information proves otherwise, I just can't see this. RJ was not his usual ambiguous self on this point.

So who do I think it is? One of the last Aes Sedai at Rhuidean. They have been shown to have taken actions to guide the Aiel towards their destiny to fulfill their role at The Last Battle. It's not a stretch their investment in them would stretch past The Last Battle by making sure the Aiel survive and flourish. This explains the guiding of Avi to shape the Aiel's direction. They created a Ter'Angreal that is able to allow some sort of time manipulation and consciousness transference of connected threads in the Pattern. This somewhat relates with Rand's manipulation of the Pattern we see in the final confrontation. So my theory is that at least one of the Aes Sedai involved with the last of the Jenn gained enough knowledge to be able to manipulate time and the Pattern in much the same manner as Rand does. Even Rand's body swap somewhat mirrors the transportation of consciousness to a connected thread that takes place in the Ter'Angreal at Rhuidean. Just instead of thread connection being that of ancestry, the connection here is from the Balefire incident. Which explains Nakomi's understanding of what Rand is doing at the very end.

I always guessed Halima used compulsion to change Egwene's opinion on the oaths. Which greatly hamstrings the Aes Sedai when dealing with Dark Friends and the Seanchan.