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.

Passive: Trample

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.

Q: Pulverize

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.

W: Headbutt

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.