Is there a slug based Sage Mode? As we know, Naruto (Jiraiya's apprentice) has mastered the Sage Mode of toads. Also, Kabuto (Orochimaru's apprentice) has mastered the Sage Mode of snakes. So, my question is, is there a Sage Mode of slugs (related to Tsunade)?
The Sage Mode of the slugs is somewhat related to the Sage Mode of Hashirama Senju (First Hokage). He learnt Sage Mode in Shikkotsu Woods/Forest, a legendary place that is equally famous as Mount Myōboku (toad) and Ryūchi Cave (snake). It is the home of the slug Katsuyu, parts of which are summoned by Tsunade and one of her disciples, Sakura Haruno. 

Evidence:

In Hashirama's fight with Madara, it appears that he enters a Sage Mode (bottom right corner) similar to Tsunade's, with a diamond shape tatoo-like symbol in her forehead.



Though it is not explicitly said neither in the manga nor in the anime that Tsunade enters Sage Mode, it is clear that Hashirama did.