I know that the theory is wrong, since it has been published, but that does not mean that many people haven't gotten any theory. Here's mine, not that indept, but I'll manage to rewrite everything in here.

My theory

Not much is known ‘bout the future of Squall and Rinoa, only to think that they will marry and have a child. I do not know how much, but they will have one, that’s a fact. Wether that child will be a girl or boy isn’t know aswell, yet I do believe it will be a girl. My own theory consist of the way Squall and Rinoa gives both their power over to their child. Squall and Rinoa aren’t young enough, nor strong enough to fight, but their child does so. They decided to give their power over to their child, becoming a strong lionish sorceress. I won’t be talking ‘bout knights, I can’t just make up a own knight, right?

That sorceress isn’t Ultimecia as most of you think…Squall and Rinoa calling their daughter ‘Ultimecia’ would be a laugh and wouldn’t really fit with reality of past, present and future. Anyway, this goes on and on to each daughter. Lion and Angel power stay’s on. What also surprises me is the fact that the story of defeating Ultimecia has stayed on as a bedtime story. ‘A lion and Angel defeat the future sorceress and they lived happily ever after’. A mere fairy tail as that.  This eventually reached the ears of the young Ultimecia. Yes, Ultimecia is in that way related to Squall and Rinoa. I doubt that she is half related, maybe her father and mother comes directly from a family with a hatred past. Therefore it is clearly seen that Ultimecia has darkness inside of her heart. She is kind of jealous of that love story between the Lion and Angel and decided to take them out after she received the power. This can answer the fact that she has Griever and wings, and maybe the kind of look like Rinoa.

I forgot to tell early that Squall may have found Griever soon after the battle, not is stated how or when. That’s how Ultimecia got her own GF as her knight, because I clearly think that Griever only obbey’s Leoharts, and because Ultimecia is half one, he obbey’s her under trance.

Ultimecia doesn’t really now the real events during the story she has heard, so when she took Seifer, Squall’s enemie, as her knight in the past, because she can’t use Griever there, she didn’t knew is was the knight Seifer in the story and she couldn’t figure out she was the evil sorceress. Ultimecia yet tried to achieve her goal, yet not knowing the truth of her power that is rather older and weaker then Squall’s and Rinoa’s.

At the battle she recented so much that she became stronger, but yet weaker physically in battle. What those words meant are her own feelings, the way everything changes through time, nothing stay’s the same. You are the white side of Yin-Yang, yet you can pretty much change soon into the dark side of it. Nobody is perfect, nobody stay’s the same. The past, present and future are always different, therefore the Time-Compression.

At the far end, she finally figures out that missing part in the story, that the end just looks the same as in the story itself…that she was the evil sorceress. To make it up with everything and to die in peace, she goes through her grand-grand-grand-grand and many –grand-father memorie, to search for someone that might take up her power, so Edea’s receives her power, to protect her children from becoming one.

There is change that many people thing that she will arise again during the future, yet when someone is defeated won’t arise again, or not in that same kind of way again. There is a slight change that they can prevend the future from happening by not giving their power over to their child, but to die with it or destroy it for greater good.

This may be my theory, but a lot of things kind of compete with question and the story. I want you to think ‘bout it, I do not ask you to believe it. Please Rinoa isn’t Ultimecia, do not believe that…

Other parts

The bedtime story has more indept information, maybe the reasons for her words at the end of the battle or why she choice that location for her castle.

Ultimecia looks alike as Rinoa's, perhaps because they are related on this way. Ultimecia has the eyes of Squall and the figurine of Rinoa.