The spoiler tag has been placed on this topic for a reason. If you have yet to complete the story please read no further.
We know there will be new Guardian Spirits on the upcoming DLC's. How many we will end up receiving is unknown at the moment but i suppose something like 5 or 6 new Guardian Spirits is reasonable. With that being said let's take a look at some of the Guardian Spirits that we know about but currently have no access to them.
The Guardian spirit of Otani Yoshitsugu that is freed from his body once we kill him. This Guardian spirit is one of the few with an humanoid appearance (the other one being Saoirse). After we kill Otani his Guardian Spirit disappears instead of joining William. We could quite possible end up obtaining this Guardian Spirit on one of the DLC episodes as Otani is the father in law of Sanada Yumikura and one of the DLC episodes is centered around this character.
William stabs and kills the Guardian Spirit of Kelley, however he ends up obtaining the power of it as he uses it to alter his appearance when he infiltrates on the castle located on London.
The name of this Guardian Spirit is unknown but we can be %90 sure that we will ens up obtaining it on the first DLC episode.
Aside from those mentioned is there any particular Guardian Spirit that you would like to see being introduced? Perhaps a gorilla,horse or even an unique humanoid similar to Janome-cho and Saoirse.