Aftershock, like Courage of the Colossus, is a great keystone choice for Alister as it gives you defenses upon engaging and a bit of extra damage thanks to the explosion.
Unflinching allows you to stick to a target while your [E] charges after Flashing onto them, helping you proc the extra stun for the kill.
Conditioning allows you to scale into the mid game as an incredibly resilient front line tank with just a few items and your [R], so that you can get multiple [Q]s off in a teamfight.
Second Wind help you to sustain in lanes where you might risk getting poked out. It also empowers your early all-ins, especially at level 2, with it's mid-fight regen.
Hexflash in an incredibly threatening ability on Alistar if you learn how and when to use it effectively. If you're able to control a lane brush with a Control Ward you present a ton of lane pressure being able to Flash out at any moment.
Hexflash is also great for roams to the mid lane where you're able to flash in the lane from either the Raptors or behind the mid walls.
Future's Market doubles down on Alistar's engage-heavy playstyle that can often lead to big snowballs, stretching out your gold lead for even more early power-spikes.