Gosh, there are so many things.......
1. Not messing up the colours on every single character
2. More background info on Zira and the outsiders. Also, add some sort of explanation as to what happened to the cub at the end of TLK which quite obviously wasn't Kiara.
3. Make Kiara more like Nala, rather than just a female version of Simba. Develop her character more. Give her some depth.
4. Better animation overall (it's not terrible compared to other sequels, but it could've been better)
5. Not change Simba and Nala's personalities as much.
6. NO TIMON AND PUMBAA
7. Score by Hans Zimmer, songs by either Tim Rice and Elton John or Stephen Schwartz, Alan Menken.....aka people who know what they're doing.
8. Overall I'd aim for the story to be told from a different perspective; add some depth and drama to it.
There's probably more but these are the main things I'd change if I were to direct a remake of SP.