Rinoa IS You Know Who!! Stop denying it!!!!!!! (Spoilers)
First off, the Final Fantasy "Ultimania" guide makes no individual or explicit comment on Rinoa or Ultimecia's ipseity, therefore the FFVIII FAQ in this forum is imprecise in stating that it is "not possible" for Rinoa to be Ultimecia. The only germane notion the guide produces pertaining to the R=U thesis is the now canonical statement about sorceresses having intrinsically normal life spans.
Now, we can all know (or at least, we can all comprehend) that when suppositions such as "time compression" and "time manipulation" come into play, any order of normal life spans based on "normal time" become obsolete and extraneous. The concept of time compression also dismantles the syllogistic argument of recursive and subsequently independent "moment-to-moment" time, because the very nature of time compression seeks to coagulate all time and all moments together, therefore instituting a universal singularity with one time and one continuity that possesses unabridged cause and effect.
So what does this all mean? It indicates that Ultimecia's leverage in Squall and Rinoa's timeline most conclusively altered their future due to the temporary temporal singularity created by Ultimecia's attempt at time compression. Because of time compression, no lateral or surrogate timelines or universes are generated and no paradoxes are encountered. Instead, we have the realization of one of man's most ancient fantasies: changing the past. When the temporary time compression phenomenon ended upon Ultimecia's destruction and time was "unfolded" from its compressed state to its "normal" state, a reverberation agent created an echo effect from the point in the time dimension signifying Rinoa and Squall's time into and throughout the future, based accordingly upon the changes caused by Ultimecia's interference in the past.
This subsequently means that if Rinoa ever was on course to become the sorceress Ultimecia, she may now never consummate this possibility because of the reverberating repercussions in the future caused by Ultimecia in Rinoa and Squall's timeline. The ultimate denouement may very well be a completely different future.
And now, following a proof of plausibility and possibility, I say to you climactically that I most surely claim Rinoa to be Ultimecia and there is nothing you or anyone else can do about it!!! (unless you work for Square Enix and are able to issue canonical, post-release plot resolutions).
P.S. Stay tuned for a proof of probability of the R=U theory!!
-LYCHON