Animal companions should be able to communicate with the rangers. Ranger and the companion should understand each other like dog understands human in real life but they should share more deep link between each other.
In the game there should be some kind of dialog (mind communication) between the ranger and companion which is more rich depending what kind of animal the companion is. Imagine e.g. event when you enter a new area and your companion stops you to warn about the danger ahead with limited description.
Companions could also provide some side quests that are only available for rangers with companions. For example companion could notice tracks which leads eventually to new quest or companion initiates NPC dialog, etc..
Some of the items could be found only by certain companions...snake will find something under the rocks, eagle something from the tree top, etc..
Companions should get also certain experience and new experience points could be used to improve such skills as scouting, search, evasion, ambush, etc.