Squall may be a descendant, husband, mind, true knight of Ultimecia. Why and how? -
Squall has expressed his sadness, anger, loneliness and non-ambitious acts through out the entire story. Ultimecia has the kind of the same behaviours as Squall execpt being more ambitious and anger, she's still lonely.
Ultimecia has created mind (Griever) at the end, Squall doesn't seem so surprised because he knew it was going to happen.
The past/present/future timelines seem to be affected at some point, for example: Squall knew something from the future timeline was going to happen but he was so totally unsure and therefore, lacked his insecurities, that's why he became surprised when that "something" (Griever) being appeared. This theory has a benefit of doubt: why did Ultimecia creat Griever from Squall's mind as the only one thing to appear in those characters' views but not something else? Why only from Squall? Why not someone else too? Anyway, Rinoa looked a lot surprised when she was asking Squall in the middle of the game about his Griever Ring, at that moment, the tune has changed to a beautiful song to represent something, I don't know what exactly. So at the end, Griever showed up, I can't tell if any of this was Squall's intention but why? It could be either Squall, Rinoa or Ultimecia (both Squall and Rinoa).
The Ultimecia Castle is located to Eden house, know why? It's because Squall has grew up living from Eden's house with Matron and spent most of times in that house thinking about things, could be bad things, so that's how the castle appeared next to it, I don't think Matron (or Edea) is involved.
Here's another theory but kind of relevant to the first theory (above). Squall has insecurities and Rinoa is more... of beauty? Therefore, Squall and Rinoa are combined into one form aka
Ultimecia as they loved each other and shared love with each other and became corrupted, since sorceresses and humans aren't exactly the same so when they shared love, they exactly became corrupted. Ultimecia's mind is Squall because of her behaviours similar to Squall, and Ultimecia's body is probably Rinoa because of beauty. When they combined, Ultimecia became worser, expressing more anger and full of changes, wanting to rule the whole world, etc.
Some say that Rinoa may be a descendant of Ultimecia, but if Squall is indeed another descendant as well, so then basically both of em are incests which made them combined into Ultimecia because it became corrupted! Ultimecia could be parted of Rinoa and Squall together
Ultimecia's goal was to rule everything, that's exactly how Squall and his team successed in fighting wars and battles, leading to Ultimecia's timeline... but for some reason Ultimecia became corrupted and tried backfired Squall and Rinoa
Oh I wanna share few more theories.
The Zell's Face Scar or rather Tattoo
Comparing to Ultimecia's face scar or rather Tattoo
Weird, eh? Both of thier face scars or tattoos look quite similar (not exactly same but very similar), I'm guessing this is because that Zell's face scars or tattoo is located in the present timeline which means the tattoo is new and well, the tattoo or face scar of Ultimecia from the future looks a bit changed and bigger, as timeline is contiuning, these tattoo or face scars grow to change a bit different.
However, Zell doesn't have same tattoo/face scar on his right side of face, I'm guessing this is because Ultimecia is as twice as powerful than Zell, therefore, she have both sides of tattoo or face scars (both right and left sides of face).
As far as I tried to recall that there's indeed none of other characters that have the same/similar tattoo/face scars as Ultimecia or Zell.
If Zell's tattoo is actual tattoo, then maybe he's interested in some part about Ultimecia? He would have thought once that if he wore same tattoo on his left/right sides of face, things would be very suspicious (for example: don't want Squall or someone else to notice the familiarity between Ultimecia and Zell) which means.... there could be many reasons why he took kind of same tattoo as Ultimecia.
If Ultimecia and Zell have those face scars, not actual tattoo then maybe that have to do with thier genes which means that Zell may be a descendant of Ultimecia since nobody else have those smiliar face scars.
Or maybe those face scars give some special power, Zell is good at fighting, Ultimecia is good at fighting either.
Too bad Zell doesn't know about this in the game, but of course the whole game is presented, in future he probably will find out.
If they were just some art creator's imagination that have nothing to do with the story, etc, then no comment about that one...