Well, I wish to coin up a new theory, here for you all to think about; It might have been considered before, so please do forgive me for a possible rehash of old known speculations about the matter:
I believe it was Zira with her cubs that were indiscriminatly banned to the Outlands because of their (and some lioness companion's) close ties to, and willing to aid to Scar's rule, handing out the so-called blankets as a co-ruling power, not only keeping the Hyena's at bay. (It must have been in that time-period she'd got her drive to kill...As she had easy targets to pick consisting of rebelious Hyena's..) But also harrassing the defiant Pridelander lionesses. Instilling a true terror, to keep the former queen and her hunting party at full speed, round the clock massacring of live prey to feed the masses of Scar's followers, and make sure the Pridelanders loyal to Mufasa's ways, and soon to be serving under Simba's reign, were last in line for dinner, whilst working under harsh (If not downright severe!) conditions.
Zira may not have been seen at the Battle for Pride Rock itself, but one can speculate she was out there in the Pridelands, probably giving birth to Kovu. In the aftermath of Scar's defeat, Zira could already be under Simba's suspision after she alledgedly kidnapped cub Shetani (Later named Vitani) from one of the loyal Pridelander lionesses to further Scar's deathwish, creating a legend around the latter, to prepare the remaining Outlanders into a standby-status for war, whenever she deemed fit to attack the royal's their territory.. Often using her cubs as ploy to get a shot at Simba himself, whom she holds for a brutal killer, denying the Outlanders their rights of decent excistance. Yet Zira probably did not know the fact, that it were the Hyena's actually killing her mate, she might have seen Scar fall from the top of Pride Rock, presuming he was dead already when he'd hit the rocky plateau, before being engulfed in a sea of flames.. (She could have identified Scar's remains by estimating the location he fell, observed from a distance, viewed from amidst the confusion of the fleeing Hyena's.
As for Kopa I believe he was attacked by Crocodiles whilst exploring the banks of the river's location. But perhaps he might have survived the ordeal, and simply never been able to return and became a rogue himself, due to his belief, that he couldn't rule the kingdom no more.. (This is based on the scéne where Kiara bumped into Kovu for the first time, and escaped the Crocodiles during her cubhood.)
But I think Kopa's dissappearance was falsely blamed upon Zira, as she was accused violating one of the most fundamental laws of the Great Kings, henceforth she was being labeled a scapegoat.. So in the end, Zira and a few loyal companions were banned to the Outlands under the pressure of false testemonies, whilst this was not known to the royal family at the time Simba exciled them. But this grave error was finally undone by (partly) accepting the former Outlanders back into the Pridelands. Yet tension kept brewing amongst the ever devided subgroups of lionesses, still excisting for a few generations after Zira's fall from power..
I believe Zira knew her chances of survival, and that she limped out of the flood at some place, wandering back into her native homelands, knowing she had survived the three rogue antagonists whom attacked her native pride, by age! I want to refer to Kovu's mere sigh after she fell off the cliff, as a signal that all was going to be allright, not just for the newcomers to the royal pride, but also for his hope of survival of his vanished mother! (There just was alot of water, and not too many logs and debris being shown in TLK-2 at Zira's presumed death.)
That's what I make of it all. Hope it helps your quest for answers! For me personally I like to speculate about the matter aswell, glad to see somebody finally brought up this topic.