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[spoilers for the latest chapters of Tsubasa and most of Cardcaptor Sakura ahead!]

So, we're in Clow now, and as it would seem, Clow's past. Imagining what could happen in future chapters got me thinking about whether or not we would see young!Touya or young!Yukito, and upon wondering that, I began to wonder about whether or not we would see Clow Reed, either.

And that got me thinking about Sakura's mother, because CLAMP is notorious for keeping lovers together, you know? Or, not necessarily lovers but soul-mates, or even just one-sided feelings.
Since Syaoran's (foster)dad is Sakura's dad from CCS, it's probably safe to assume that he was at one point married to Nadeshiko as well, unless she never existed in Clow's world, and if she did exist, maybe she died in this world as well.

But if Sakura's parents from CCS are Syaoran's foster parents in Tsubasa, and Clow Reed is now Sakura's father, then who do you think is her mother?

Like I said above, CLAMP is notorious for keeping couples together. If Clow Reed is Sakura's father, her mother would definitely be someone he was in love with, or was in love with him. And if that's the case, it could easily be one of three people.

The most logical assumption we can make is that Sakura's mother is Mizuki Kaho (and I say this with gritted teeth, because I admittedly hate Mizuki Kaho).
The fact is, Miss Mizuki is definitely in love with Clow Reed (or Eriol, as the fact may be) in the CCS series, and the feeling is definitely mutual.
As she seems to get off on the little boys, she was once in a relationship with Touya, but broke it off in an uplifting way shortly therafter ("The next time we meet, we'll both be in love with somebody else.") and does indeed make good on her promise, as the next time they meet he is attached at the hip to one Tsukishiro Yukito and she is likewise trailing after Hiiragizawa Eriol, an ancient man in a ten year old's body.
Eriol and Miss Mizuki eventually run off together to England, and this is apparently okay with the school district, probably because Mr. Terada can say nothing about child mollestation without sounding like a hypocrite. Oh, CLAMP. sweatdrop
Point being, Mizuki Kaho is a safe bet to make. Not only does she have a very real relationship with Eriol, she's also largely a mother figure to Sakura in the series, most notably shown through Sakura's dedication to writing the woman detailed letters in a similar fashion to how a little girl would run home from school and tell her mother about her day.

Aside from Kaho, there's also another woman in Clow Reed's (first)life, a jaded lady who goes by the name of Madoushi. Admittedly this wasn't a very healthy relationship, and Clow Reed leads her on in a way he's very good for doing to many people, apparently. (Hey, maybe this is why Yuuko hates him so much..!)
Madoushi doesn't show up in the manga but she is the antagonist in the first Cardcaptor Sakura movie, which starts with Sakura winning a trip for herself and three others to Hong Kong. She of course takes her brother, her brother's boyfriend and Tomoyo, and as things go, they meet up with Syaoran and Meilin while they're there. Go figure.
Madoushi is a villain. She had faulty ideals and Clow blows her off because of it, but she was genuinely in love with him and genuinely broken up about the fact that he is long dead when Sakura shows up. Besides this fact, the thing that makes her more likely to be Sakura's mom in Tsubasa is the fact that we have yet to see a mother yet, and Madoushi does indeed die in canon. If Kaho were Sakura's mother, where has she gone?

The third and least likely but still possible mother to Sakura might even be Yue (and as I said Kaho before with gritted teeth, I say Yue with a huge smirk because pregnant Yue? HILARITY!). Yue in CCS was asexual; he was a creation of Clow so he therefore had no gender. However, Tsubasa made Chii into a real woman (before she died and Fai created a simulation robot of her) with the real capacity to become pregnant with Yuui and Fai, so who knows? Yue reborn in other worlds could easily be reborn as a woman.
The reason I even bother stating Yue as a potential candidate is the fact that Yue had real feelings for Clow Reed, and though he was shot down in the same way Madoushi was shot down (which is so unfair, he wasn't even evil.) he was still in love, and that counts in CLAMP-verse.
I'm not sure if Yue and Yukito can exist as seperate entities, but like I said, it's not my first guess.

Those are three of Clow Reed's canon love interests, but it doesn't have to be that Sakura was born from a loving relationship. It might even be that Clow Reed just knocked up some woman to create an heir to the throne, but since Touya was born before her, I don't see why Clow would try for another boy (unless he somehow knew that his first born is gay and not gonna give him any grandson heirs anytime soon).
Going on that, you could try and speculate from Sakura's point of view. She had a lot of female role models in CCS, including Syaoran's mother, Yelan Li and Tomoyo's mother, Sonomi Daidouji.

SO, longass post aside, anyone wanna venture a guess?

I can't imagine either.

LOL mystery solved~!
But I'm still super confused over what the heck is going on, being that the king isn't, you know, who he is in the beginning.

Sakura's mother is Nadeshiko. It's revealed in the latest chapter.