I guess one question, which has already come up in this thread once but wasn't specifically addressed, was which coin she would have had.
Lasciel's is the obvious answer, but maybe not the best.
Rosanna's coin is another interesting option, but was presumably in use by Rosanna when Margaret was active. I don't know if we know how long Rosanna has been Tessa's second, but I was left with the impression that it had been a while.
And then there's Thorned Namshiel. Again, we have no way of knowing how long the previous host was in possession, but Namshiel being a caster coin makes it an interesting pairing.
The rest aren't particularly special, from what we've seen. And there's the possibility that she had one that's still secure somewhere and out of circulation.
The thing that appeals to me about Namshiel is the idea that Namshiel's host ended up infected, with Namshiel either going along with it or being helpless to stop it.
If Namshiel went along with it, it makes you wonder about this rebellious streak that the other Denarians didn't seem to know about. Given Margaret's rebellious nature, it's an interesting pair, and makes me wonder about the possibility that she influenced Namshiel somehow.
If Namshiel didn't go along with it, but lost control due to infection of the host, it makes you wonder who the host was, and when they were infected. With Margaret potentially being involved with Outsider business with Raith and the Sidhe, could it be that she gave up the coin, and someone else involved (and infected) picked up the coin?
Not to mention the potential for familiarity with Winter, if Margaret was involved with Mab like some theorize. It'd make the attack on AT more interesting.
Of course, Lasciel has a lot of interesting possibilities, like Lash knowing Margaret's history, but there's also the issues, like neither her nor Nico using that fact to help recruit Harry. Seducing Harry with the promise of info about his mother would have been incredibly tempting. Offering to appear as her, or share the memories of her life with her, would have helped Lasciel seal the deal.