After installing the expansion pack you will gain access to an additional Traveler skill tree. As the name suggests, the skills are mostly focused on the relation between Aloy and her mount.
Mount Repair and Machine Repair are especially worth attention. As the name suggests, they enable you to fix the overridden machines with metal shards. This enables you to minimize the risk of losing your machine during a critical situation. The Dismount Strike is worth attention as well. It enables you to perform an attack by jumping of the mount. The damage dealt in this way is quite high. In case of smaller machines like Watchers you can kill them with one blow.
As for the skills unrelated to the mount, you should definitely invest in Hoarder, which increases your supplies capacity by 20%. Shard Salvager which enables you to disassemble resources is useful as well - it enables you to reclaim 50% of the value of an item. It's useful when you lack enough shards for something.