I did start writing one. I admit, I haven't seen the Lion Guard series yet, but I have seen 1, 2, and 1 1/2.
I had this idea to make Shenzi the main villain in this one, Lion King III: Return of the Hyenas.
Basically, Shenzi gets the idea to get rid of both Simba and Kovu, as she hated both Mufasa and Scar. Shenzi would be married to Banzai by this point.
Shenzi, Banzai, and Ed wouldn't be the only bad guys. I recall from the original Lion King, that, when Mufasa and Zazu were discussing Scar, Zazu said "There's one in every family. Two in mine.". I got the idea to bring in two evil brothers of Zazu's, who would want to be the new adviser to the King and Queen of Pride Rock. Much like Scar, they are perceived as annoying and sinister but nobody perceives them as an actual threat dangerous enough to act against.
Eventually, these two would meet up with the hyenas and cook up a scheme to oust the lions and get the hyenas in charge, where the two evil hornbills would replace Zazu.
They'd recruit jackals, desert cobras, and scorpions to join them. They'd start dissension against the lions. However, Zazu's evil brothers wouldn't be tied to it (at least, not until the hyenas were able to take over).
The Hyenas said that Scar had used them and that it was the Lionesses who overhunted the Pride Lands and that the hyenas cannot be blamed for the stupidity of the lions. Also, they'd blame the mess with the Zira vs. Simba stuff on the lions too.
During this time of tension, more conflict would come as Timon would fall in love with another meerkat named Tíma, and his relationship with Pumba would suffer.
Also, to make matters worse, Kovu would be getting visions sometimes of Scar (much like Simba did of Mufasa) and Scar would say that he'd been like a father to Kovu and keep demanding that Kovu avenge him, saying that Simba still killed him and that he needs to avenge him.