The only reason that people who try turn into toads are because they use the toad oil. If they were to put on snake venom instead, I believe they would turn into a snake. As to an improper snake sage form, I believe they would probably begin to sprout scales all over their body. As to abilities, possibly able to sense chakra with their tongues, that would be elongated. Eyes would have that glass over look similar to most snakes. Possibly the abiity to slither/move more silently than the stealthiest ninja. An extra tough layer of skin that they shed when the sage mode time limit is up (sortive similar to Gaara's Second skin). Elongated fangs with venom.
WHAT DO YOU GUYS THINK?
I like the idea for my own personal reasons though. The storyline that I want to build for my character will eventually allow him to summon a certain animal (I'm not saying because I don't want it stolen.) Its not listed as of yet so I'm gonna wait til I can make my own custom animal with my own custom sage mode. I actually had that idea before you posted this. I felt obligated to post on this thread. lol
But I will give hints. The sage mode of my animal will give me strength similar, if not exceeding that of Tsunade and Sakura. The will to never give up even if mortally wounded (wouldnt call it will, but its damned near close to that word. lmfao). My improper form will sprout an extra appendage. It has been used in Naruto only 1x. What is my animal?