What they did with Ward in Shield was brilliant. Anyway if he did turn evil it doesn't have to be out of greed people can fall to darkness for noble reasons, though another thing they could do is have Kion become the antagonist without him becoming evil at all instead he and his family just split over differing ideals for the future of the Pride Lands perhaps through the guard Kion develops a progress ideology believing that animals other than lions should determine the future of the kingdom maybe through the guard Kion concludes there shouldn't be a monarchy which goes against Simba who believes the kingdom isn't ready to rule itself and fears it will cause chaos. Kion believes in progress and absolute freedom this is noble but could potentially cause a lot of chaos. Simba believes in tradition order through strong leadership this is noble too as the past shouldn't be forgotten though this runs the risk of eventually creating a tyrant.
Neither side is wrong and it could be a truly intriguing conflict. Kion's not evil and I feel this kind of ideology wouldn't completely go against his character either while Simba's motivations would be completely understandable too. Naturally I'm really grasping at straws Disney has always been all for good vs evil it's highly unlikely they would ever do a story where it was next to impossible to determine who was the good guy and the bad guy, even more so a conflict where neither side is right or wrong, kind of hard to teach a moral in that kind of situation. Really I know this is unlikely but it's fun to speculate no matter how unlikely it is.