Maokai's Ult + Movement Speed
If anything, the issue with Maokai's ult is how slow it is. Because of its speed its been strongly disliked by the community even though it looks great.
So I'm thinking, why not have Maokai's ult scale with Maokai's movement speed? Yes he's a slow tank, but tanks still build tier 2 boots and a lot of them build Dead Man's Plate. The current ult could be the minimum movement speed (so it can't go slower if Maokai is CC'd) and have it go faster from there based on his movement speed.
Other than its speed, the ult is fine as is. 2.4 second root is a strong form of CC, it just needs that little bit extra to make the ult feel worth it.
and his new E. At least until people know not to facecheck Maokai bushes.