This is what my thoughts went to immediately. Something like this:
Events of Yesterday's Enterprise sends crew of Enterprise-C to the past/back to their original point in the timeline. After the battle, the crew of the Enterprise C are taken to Romulus, tortured or killed etc? We know the genesis of Sela, what happened to the rest of the crew that survived the battle of Narandra III?
The survivors might have been locked away for decades. Tortured or experimented on. They may have come to hate Picard/The Federation for their suffering. Despite the altruism of their actions (Klingon war avoided). Over time, they banded together and escaped Romulan captivity (or possibly ascendants, given how many years passed).
Now these people have a huge axe to grind against Picard for placing them in their predicament, and the logs of the Enterprise D, a matter of public record, have become like a road map for all who want revenge against Picard. We know that history recorded the battle of Narandra III as a noble sacrifice, but we don't have the other side of the story. Through the logs, Vadic and company they start making a master list of all the top baddies Picard/The Federation at the time faced. Moriarty, Lore, The Dominion.
Vadic talks a lot about patience with her attacks. Maybe this is generational. Kind of reminds me of a Khan type cult with an objective. Stick around for decades until all the pieces are in place and let loose. Keep an inner circle of shapeshifters to insulate against threats. There's no chance Picard and crew can handle all these baddies at once (evil cackle)
This means Tasha or even Sela doesn't really need to appear onscreen - but still have a huge impact/connection on the crew as they try and figure out that their actions - in another timeline - have created this major villain that rises from the sum total of their adventures (the Captain's Logs are the key!)
Scooby-Doo 2 Monsters Unleashed meets the TOS films with the TNG cast.
Predictions:
Lore rescues the Titan A in episode 4. Leads them into...
Moriarty trap starts in episode 5. (Trippy ten forward on the ENT-D?)
Moriarty trap escape in episode 6. (Assuming they have access to a holodeck in this nebula)
Tasha Yar connection to Vadic in episode 7? Reveal flashback how she got command of the Shrike
Episodes 8-10 deal with Poppa Picard learning things about fatherhood maybe and then a big pew pew battle with every Enterpise ever. Picard dies for real, because he just learned about fatherhood and what better time for Patrick Stewart to exit the role?
"Nothing but blue skies..."
[/wild rant]
PS I am enjoying the suggestions about shapeshifters, but the core of the story now seems to center on Picard and Riker's points of view on fatherhood and I don't see how that lands if Riker is a shapeshifter. At some point, when one of the TNG cast is duplicated, I think we the audience will be in on it from the start.
