did I just see Cassandra in the background?

At 1:59 ? Yeah, that does look like her.

Yes, Kassandra was shown in the "Deep Dive Trailer" as well. Without going into too much spoilers, I'm going to spoil a single part of the ending of Odyssey, I've put it in spoilers tags, so anyone who doesn't wish to know can avoid it:
Kassandra becomes semi-immortal with one of the pieces of Eden, so she - he, if you played with Alexios - could appear in basically any Assassin's Creed game made from now until the present timeline. Also, they could make her be the unseen architect/sponsor of the Assassins Brotherhood if they want.

Having explained that, that's almost certainly Kassandra. The only question remaining is if Kassandra is now canon - which she totally deserves to be - or if the game will check your Odyssey's saves or ask you which of the two you played in Odyssey.

I usually check Reddit and the Ubisoft forums after a bunch of info gets revealed, but I'm not even sure I want to watch or see anymore, since I already know I'm going to play it. Therefore, everything new is basically a spoiler. I'm actually pondering whether to pre-order just the base game or the Ultimate edition. Or to even wait 3 months after the game's release to buy it heavily discounted.

It will all depend on PlayStation 5's price and its price in my country, as well as Nvidia's and AMD's new GPUs and their prices. I've been saving to buy one or the other, but the Covid-19 situation made prices soar absurdly high. Specially since almost all sellers are modern-day legalized thieves - most used stuff is selling as high or even higher than brand new ones, after the retailers had and are still using the pandemic as an excuse to hike up the prices 200-300%.

Anyway, the game looks great. But if anyone can find the leaked gameplay videos and can watch the extended demonstration shown in the post-show of the Ubisoft Forward event, please notice the music. It's brilliant and some tracks give strong The Witcher 3's Soundtrack vibes. I'm glad they're releasing some tracks tomorrow, but I hope someone at Ubisoft Marketing understands the gold mine they're sitting on and release a 5 minutes long developer look into Valhalla's music, the tracks I've heard so far are utterly brilliant and that aspect of the game deserves to be highlighted as much as possible.

Also, if one isn't interested in the game, I would at least urge them to listen to the soundtrack and after the game's release, if they like it, to buy them as the artists have certainly outdone themselves.

My bad then, but is it her
daughter/descendant
then? Or Ubisoft just decided to make an extremely similar NPC to Kassandra or reuse her character model asset? So confusing... Oh well, either way, we know
she's alive throughout all the years till the present modern day time, so Kassandra could still be in it - even in flashbacks.