I'm late to this thread...
Potential FM already undercover... Ser Shadrich, the Mad Mouse. Mayhaps he is who he says... but maybe he's working for Illyrio. If he does find Sansa, he can switch guises and escape with her.
And a recently deceased character ready to be impersonated, I suggest... Ser Kevan Lannister.
Varys murdered him in secrecy. If Varys is a FM or has access to FM, Kevan could be impersonated.
Kevan's death most benefits Aegon.
If Kevan is impersonated, then he can continue to undermine the Lannisters and their alliances. And then pull the rug out from under Mace when it will do the most harm. Kevan is one person who has total access to Tommen. He can also easily gain admittance to see Cersei... and Jaime and Myrcella if they ever go back to KL. Kevan would even be entrusted with a high command in the defense of the realm... a position from which he could easily allow Aegon to win a victory.
Conversely, they Tyrells stand to also gain from Kevan's death. Sure, Kevan invited them to take the reins of the government, but eliminating him means that it will be nigh impossible for the Lannisters to exert any control or protection over Tommen. So, if Olenna, Mace, Margaery, and Loras are still supporting the Targaryens (i.e. hoping to marry Margaery to Aegon or either Willas or Loras to Dany), then they can easily present the returning dynasty with a fait accompli. Impersonating Kevan until the time is ripe just keeps the Lannisters off balance that much longer.
But if Varys is acting on his own without thought of Aegon, then he can continue to keep the realm rudderless by impersonating Kevan. Mayhaps Varys and Baelish are in this together.
Along these lines, I think that a possible face for Arya to assume in order to kill Cersei is Tyrek Lannister. He's the son of Tygett, another of Tywin's brothers. He and his cousin, Lancel, were squires to King Robert. He disappeared during the riot after Myrcella's send off. If he were to return, he'd be close to either Cersei in Casterly Rock or to Tommen in KL... or maybe close to Ser Daven... or he might be sent to serve Jaime, if he ever returns. Impersonating Tyrek would give Arya almost unlimited access to the Lannisters.
One more thought regarding a potential FM... We assume, at least I do, that Balon Greyjoy was assassinated by a FM. I assume that Euron had his FM dropped off on Pyke at night and then pretended to arrive the next day.
There is a certain amount of skill and insider knowledge that a FM must possess to fit in as an Ironborn... and I do not think that disguise would hold up for long on a reaving ship. The escape from Pyke needed to come as someone overlooked by the authorities... a salt wife or a slave.
If Euron was willing to use a FM once, why not twice? But what could be as important as gaining the Seastone Chair? His bride... the Mother of Dragons. But he entrusted this mission to Victarion... the thing is... he does not trust Victarion.
Euron knows the fleet is loyal to Victarion. Euron knows a FM cannot pass for an Ironborn on a ship amongst real Ironborn over the course of a year. But he needs a spy and mayhaps an assassin to deal with Victarion if he proves treacherous...
The dusky woman. Euron said she was captured off a slaver out of Lys. He said her tongue was cut out. She is beautiful. Like Ser Ilyn, she is a perfect listener. As Victarion's woman, she sees all his moods and hears his plans. She has access to him when he's injured and when he's asleep. And she does not have to mingle with the rest of the Ironborn.
