Problem is that riot pretty much decided that Alistar is a support. He will always be a support and will be balanced around it unless they fully rework him.
The passive you are suggesting would work wonders for a mid/top/jungler but it would give a support too much damage: alistar doesn't benefit too much from defenses so he prefers sheer HP so he would easily reach 3k health right? Then he would easily reach a free 60 damage per second at that point (even building ap tank he can easily reach the 3k hp). And he would also get free movespeed which is always good.
I do love this (I always wanted Alistar's passive to be almost exactly like this, only scaling with health and ad) but it is too strong on his kit.
The Q while I do like it, making a skill scale on an opponent's stat (unless it's hp) it's never good, it's unhealthy and unfair so at this point they shoulod just change it into an AD scaling, would be far easier to balance and fair.
The e is problematic though:
Alistar Heals himself and nearby allies targets for 30/50/70/90/110 +9% of thier missing health and 6% of thier bonus health. Minions receive 200% of the base healing.
The problem is that it's a lot, also the healing shouln't be based on the ally, but on Alistar's bonus health to be more fair else he would heal tanks too much and squishies too less. The % missing health is good but should scale instead, else he wouldn't really have a reason to level it early and would get far too much power so something around 5-10 based on skill level would be better. I wouldn't make it more powerfull on minions though, it would be useless power. Would like it to be more spammable instead, it's better to use it more than to have it work better on minions, at least you can heal yourself and your allies more.
The R is cool, but the silence doesn't belong there, it's already a powefull tool on it's own that grants as much as 50% ms and 65% damage reduction. Instead I would give him double damage passive, makes more sense.
All in all I do like your idea and I do think it might become great if worked on but like I said, as long as Alistar is seen as a support, because he's got A LOT of gold in his own kit, they can never buff him, and these changes would be straight up buffs to Alistar in every role ^^'.
