I've ended up following a lot of Lion King artists lately over on DeviantART so I gave a bit of an attempt at drawing the LIONS - Need to work on head / eye ratio and the different kinds of noses

Everybody got theories on how Kovu ended up being Scar's heir - mostly 'cause they had him mention that Scar wasn't his dad very briefly in one scene, though easily missed (there was also a scene that didn't make it into the film that said it more clearly https://www.youtube.com/watch?v=kxVo_0LTaPw )
SO
It's probably been done - lots of theories out there but here goes;
<i>Most is explained on the image - Zira has a third cub which is weak and female, not an ideal heir (thinking here maybe she was told to not return without an heir or something 'cause Scar's a power hungry douche) and on her slow plod back to PrideRock she finds dead lioness and living cub born roughly the same time as her own. Yeah, it's Kovu but he's got no name here yet so she swaps the cubs with each other (as Kovu could pass for a son of Scar and she'd be able to return).
Upon returning to Priderock she presents the boy cub to Scar, who's happy and calls him Kovu - after Scar goes out to find Sarabi and criticise the hunt (or lack of it) Zira and Nuka have an argument about the Heir to Scar (Nuka thinks it should be him 'cause he's oldest - Zira says Scar chose Kovu and Nuka was not ideal for it etc) and during this argument she blurts out that Kovu isn't theirs - Nuka would then say he would tell Scar about this, thinking this would give him the title of Scar's Heir for sure - This would all take place on the same day the storm starts to brew and Simba returns to the Throne.
With Scar dead and his loyal lionesses exiled, Zira still has Kovu as Scar's Heir and Nuka's dislike of Kovu is pretty much solidified then, whereas Vitani doesn't really care that much (she and Kovu got along in the films so it explains that at least)</i>

Not sure how Kovu found out he was adopted yet, probably Nuka telling him in another Heir argument for something but this is what I have so there you go :)

IN REGARDS TO NAFASI;
<i>After having been swapped out she's left by a dead lioness with a storm rolling in - not looking good - the mate of dead lioness ends up finding them both and, after mourning the loss of his love (blaming himself for not going with her because Scar's Reign has ruined nearby Prides etc), takes the daughter back with him - he names her Nafasi and once the True King returns to Priderock, nearby Prides return to normal.</i>
Went to give her the scruffiness that Nuka has to further show relation - I don't have a personality set for her, very likely timid

--IF SHE HAD BEEN TAKEN BACK TO PRIDEROCK--
<i>Scar would've been pissed and likely have told Zira to leave after failing him for so long (or something like that). Arguments etc until Sarabi's Hunting Party bit starts that same day - insert end of TLK here - and things with Zira would've been much different.
Nafasi would've been named Tatu (other names were in the works but Tatu seemed fine) - she would've recieved a LOT OF CRUD from Zira about being the third strike with Scar and would likely get along with Nuka (mostly about the whole "Heir" thing and general lack of acceptance from their mother) - not sure about the relationship with Vitani though, probably ok I guess.
As for Kovu, he would've ended up with his real dad and likely been introduced to Simba as part of an allied Pride - i still think he'd end up with Kiara in this sense but most (if not all) events from TLK2 would not have happened.
I guess Zira would've tried to use her kids to manipulate Kovu into being paranoid about Simba's capability to rule or something but I never thought much on this line of thought.</i>

WALL OF WRITING, DOOT DOOT - also forgot to upload this here when I put it on my DA so here's to me being a walnut :)

TLK, TLK2, Zira, Nuka, Vitani © Disney
Nafasi / Tatu © me