(Sarcasm Mode Activated) I love how a thread about Sage Mode end up talking about Kaguya, then of the Naruto Project. (Sarcasm Mode Disactivated)
The Six Path Sage Mode is not a traditional Sage Mode, because it can not be achieve through trainning, and is unique to Kaguya Inc and temporary Naruto and Sasuke (and the Bijuu, Jinchuriki, I guess). It came from the Shinju and by extension the Juubi, and his "forced" children (Kurama at least).
Sage Mode are "transformations" achieved through absortion of Natural Energy and can be learn by trainning.
I like the idea of multiple Sage Modes, each unique to their origin. I also like to think that every transformation is like to the environment they are learn.
Naruto and Jiraya are the Toad Sage, trained by Toad. Jiraya controled his imperfected Sage Mode to change some part of his body in frog limbs. Naruto can not do so.
Kabuto (and orachimaru ?) learned senjutsu in the Snake/Dragon's Land (Don't remember its name) so he became a dragon, with all the ability link to the species .
Hashirama is Naruto's Bouddha (Shaka is that you) so his senjutsu is the Inner Eye (Insight). Sooo, i guess this is just to strenghten the character's image and link to Bouddha mythos. I have no freaking idea from where do it come from. (if someone have a theory I would like to hear it).
Juugo is, as you know, a passive user of this technique, but like Lee's Drunken Fist, it can be replicated, so it's not unique to him.
I assume Juugo began to use his sage mode in the "normal world", that is why its tranformations are ever changing, and look like weapons or monsters. He did not have a specific "model" so his powers take the frightening sharps, to scare the predators and protect Juugo (or mass killing people).
I think the Cursed Seal state 2 look like monsters, a succubus, I think (Tayuya), a hannya (Sakon and Ukon), a vampire (Sasuke), a tsuchigumo (Kidomaru) and, Jirobo and Kimimaro whom I don't know yet. I guess Jirobo looked like a Oni but he lacked the horn(s), and Kimimaro, a Dinosaur.
If a Boruto manga is created (please Sheishi Kishimoto do not let your brother do it, do it your-self), I would like to see a Slug Sage Sarada (Acid generation, and the ability to create chakra receptor seal to create a long-rang heal), and a Cat/Felis Sage Mode (my spirit animal).