I don't know if this is in the right section. Please correct me if I'm mistaken. This is something I thought up after the 2009 movie came out:
Star Trek fans in general, myself included, have often tried to make sense of Star Trek and attempted to construct a single universe out of its various incarnations. For my part, I tend to consider minutiae such as starship scaling, prop or stylistic changes and such things to be interesting but ultimately unimportant. What matters is the plot, the narrative of the Trek franchise. Unfortunately there the principal problem of Star Trek lies, mainly because of its longevity and the absence of a "writer's bible". And so one of the tasks that Star Trek fans give themselves is to weave all those contradictory events and references into a coherent continuity, but that task has not been an easy one.
Despite obvious contradictions or depictions of the past that didn't quite match their previous descriptions (Cochrane's character in First Contact, the technologies on Enterprise and the early contact with the Klingons, etc.), we have been reluctant to accept that those time travel events could really alter the timeline, mainly because we are simply used to thinking of time as a linear thing, and therefore assume that changes to the timeline would mean that what happened in Star Trek "before" that point will no longer occur; but also, because we want to be able to construct, for instance, a lineage of Federation starship designs, something that would otherwise depend on what version of the timeline we are observing, and which would become very complicated.
But what if time in the Star Trek universe wasn't linear ? If we loosely follow the actual production of the show, we go from TOS and the movies to TNG/DS9/VOY, then after First Contact we go to Enterprise's 22nd century, and now in 2009 we're right back where we started. But instead of thinking of all this as predestined, one way to look at it is that Enterprise is a direct consequence of the changes in First Contact, and that Star Trek (2009) results from changes further down the timeline, but without the resulting paradox. Of course, that wouldn't work in linear time, because there would only be one timeline.
Well, I propose "planar" time. A 2-dimensional timeline, or time tapestry, if you will.
Each time one travels to the past, the previous timeline ceases to progress, and the "new" one proceeds from that point on (2nd dimension). The events of the previous timeline are essential to the change itself, so they must have occured, but the tapestry follows the time traveler and proceeds anew from there. And if one later travels even further back, the shift occurs in the opposite direction in order to allow revisiting that past. Since only one "version" of the timeline exists at any given time, it avoids the violations of the 1st law of thermodynamics that diverging timelines would entail (making a great number of new physical universes being born every second).
Planar time thus allows us to take into account the changes to the timeline while still acknowledging the events of the past, eliminating the contradiction. We will posit that people or objects displaced a certain "distance" from their natural time period are immune to the changes in the timeline. This removes the threat of paradoxes, explains Guinan's temporal awareness (her own displacement perhaps caused by her time in the Nexus), and allows 31st century humans (temporal cold war participants) to use it to shield themselves from changes in time so as to be able to correct them if needed. We will also posit, as we need to under any time-travel mechanic, that once an object or person returns to its "own" time, they replace their duplicates in the new timeline, one way or another.
From a "franchise" point of view, planar time allows us to follow the narrative of Star Trek itself, rather than attempt to build a single, linear chain of events.
Most time travel episodes in Star Trek are inconsequential, so the "simplified" timeline above ignores all but the most significant ones, and one could assume that even those changes could be more-or-less canceled out by the events that weren't changed, somewhat diminishing the impact of the changes as time progresses again. Travels to or from the future or those with possible effects on a very short time period are also ignored for simplicity. The "complete" timeline below integrates all but the least significant events, but is presented in a less detailed format.
(These two pictures are hosted on my own website)
Phew ! Let me know what you think. This may or may not make sense.