Sage modes that would be cool but probably won't happen
To be honest, the top 2 sage modes I would absolutely LOVE to see are either Dog, or Wolf Sage mode.
Kiba would probably excel in Wolf Sage mode, since he's capable of performing a giant 2-headed wolf tranformation, while Kakashi would probably have Dog sage mode because of his ninkins.
Others are Planetary sage mode, which would be like God mode basically. It's kind of like Toad sage mode, except instead of gaining STRENGTH from Nature, you gain control over it. This means you for a short amount of time would gain control over water, earth, wood and the plant kingdom.
Bobcat and Rhino sage mode would just be overkill, if they existed Toad sage Naruto would be toast.