Alistar doesn't need a rework. However, I think these changes will make him viable for solo lane and more ultility on his kit.
Alistar's main role is Tank but I think he suits Support more. His kit is built around the ability to protect a certain point from accessed with his Q and W. His heal is in the kit when he face poke-champion. These changes will make him more aggresive while make his defense less guaranteed.
After using ability, Alistar gains 10% Movespeed and a stack of Trample, which stacked up to 3 times. If Alistar has at least one stack of Trample, his next basic attack deals bonus magic damage equal to 10 (+5 per level) (+ 15% AD per stack) (+10% AP per ultimate rank).
Max damage at max stack: 100 (+45% AD) (+30% AP) bonus magic on-hit damage.
This mean he can now be better at trade off solo lane. When he duo lane, the passive will stack and will interact with his W.
Alistar slams the ground beneath him, deal 30/55/80/95/130 (+40% AD) as physical damage to nearby enemies.
After 1s, Alistar slams the ground beneath him again, deal 30/55/80/95/130 (+40% AP) as magical damage, knocking up enemies around him.
This give the enemies more time to avoid his CC.
Passive: If Alistar has 3s stack of Trample, his next basic attack also knock his target 400-unit back.
Active: After triggered, his next basic attack gains bonus range and Alistar charge toward his enemies, deal magical damage. This attack apply all on-hit effects, and also penetrate the target armor by 35%.
Bonus range: 200/250/300/350/400 (for a total of 325 - 525). Damage: 30/60/90/120/150 (+50% AP). Cooldown: Reduced to 12/11/10/9/8.
This allow Alistar to suit solo lane moew while still keep what makes he a good support in duo lane. His Q-W combo is now easier to use if he is solo lane.
E: Triumphant Roar
Almost unchanged. But the double heal will transfer to the lowest health champion ally nearby. If click-on-self or there is no nearby ally, Alistar will take the double heal.
The new E change will heal ally better in lane.
R: Unbreakable Will
New passive: Add 10% AP bonus damage to Trample attacks.