So do you think they would add depth to the gameplay? Or would it dilute what Dark Souls is about?
Personally, I think it would work very well. If you added a sense of loss to a pet or summon, I feel it would be a great edition. You would have to maintain it's health, and use your own souls to level it up. And constantly making sure it was healthy and maintained would make you feel like you have a purpose. You're looking after something that can't defend itself, and training it to help you. This could work in a variety of ways, like pets or summons having different skills and attributes.
For example, a bear would be able to take a lot of hits, and dish out a lot of physical damage. However something like a crow or a dog would be more of a flanking offensive, or would provide some sort of technique, like a screech to disorient enemies or a bark to distract them. If you've played Pokemon, you know how it feels to raise up a level 5 starter to level 60 and it being your best fighter.
Obviously this would need a lot of balancing to make it viable, but not overpowered. Since there are confirmed woodland areas, there will obviously be animals you have to fight. But if you're something like a druid, you could gain it as an ally and summon it at will. As something like a hunter, you could trap it and train it to obey you.
If the pet or summon is killed, it could be permanently dead. OR would require a massive amount of souls and items to do a ritual type thing similar to reversing hollowing with humanity. If you couldn't do that, there would be a gravestone/sign/memorial placed where it was killed, or in the "sanctuary" type area like Firelink Shrine.
Either way, I honestly think that it would bring a new layer of depth to the already amazing atmosphere of Dark Souls.