Something that terraria did not have that I would have liked to see. You can have starter items that summon creatures that don't do much damage, then in later harder worlds you can find versions of these items that are much stronger with better effects and damage / defense. Different creatures to summon with a variety of types would be nice.
I was thinking that you'd be able to bring along with you creatures that you've researched and captured (like pokemon) instead of summoning items. This would also bring about a nice bond between you and your creature, because it's your pet that you tamed, as opposed to a creature that was called through an item.
As Kiofspa said, it's already confirmed that you will be able to capture animals and make them your pets, and train them and all, as far as I know, so summoning would not only be redundant, it would also kill the pet mechanic (why go through the trouble of capturing and training one if you can downright summon one out of thin air?)
