In reverse chronological order:
Although one person has agreed so far, I'm under no delusions that my idea will get voted in. I just like giving unconventional suggestions that start discussion. And you guys may not believe this because of how stubbornly I've defended my position, but I will change my mind if I'm given a convincing argument.
Edit: I will also edit my post at the top of the page to fix the issues that Zephias has brought up
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Congratulations on quoting wikipedia at me. Allow me to use your own post against you. According to the current Oxford dictionary, this particular use of ward would mean "to guard, to protect". Not to purify. Your wikipedia article also confirms what I was saying of Shimenawa being used to delimit sacred areas.
You note that the talk about yorishiro also very clearly point out that the trees are inhabited by Kodama and cutting them was to bring misfortune, not anything about preventing possession, purifying the Kodama. Which makes sense since Kodama are benevolent spirits.
If you're willing to fly all the way to Colombia and come to the local Tenrikyo temple so that I can have Takumasa explain this to you personally, I will gladly take you to the source of this explanation.
Where did I say that "ward" meant to purify? "Shimenawa are lengths of laid rice straw or hemp rope used for ritual purification in the Shinto religion." While I don't know exactly what Shinto purification rituals specifically entail, I'd imagine they are a ritual used to purify a person or object through the removal of a malevolent spirit or something similar. While this ritual may not be performed on trees in the real world, I don't think it's much of a stretch to imagine this occurring in a world where some trees are conscious beings. So if the prevo is a being inhabited by evil spirits and then the process of evolution represents the purification ritual, then the Shimenawa now represents that Jumbao is now inhabited by benevolent spirits.
So unless I'm way off in my understanding of what a "purification ritual" is, then I think it makes sense to combine two of the aspects of what a Shimenawa is used for a Pokemon that exhibits characteristics of both a tree and a conscious, humanoid being.
Edit: I'll admit, you've made good points and got me to slightly change tack in this post.
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I really cannot disagree more about the Grass/Ghost thing, especially seeing as the reasoning seems to be the Shimenawa. Allow me to share a bit of shinto culture with you all.
Shimenawa are knotted ropes used to delimit the separation between mundane and spiritual. They're used in Torii gates and rooftops mostly, and sometimes trees. This is not to protect the tree from possession or spirits, it's either a) to delimit the area of a temple or b) to mark the tree as one that should not be cut because it's inhabited by Kodama and cutting it can anger the gods.
There's no "purifying the spirit" in a shimenawa. Heck, if anything it points out to Grass/Fairy being a stronger choice.
On a more serious note, Grass and Grass/Fairy sound about right.
I'll admit I know practically nothing about the Shinto religion, I was just basing it off of what Pipotchi said when he submitted his artwork. Now, you may be an expert on Shinto or whatever, and if you are then I'll concede my reasoning is faulty if you can also prove to me that this wikipedia article is also wrong.
https://en.wikipedia.org/wiki/Shimenawa
Some excerpts:
"
Shimenawa (標縄・注連縄・七五三縄, "enclosing rope") are lengths of laid
rice straw or
hemp[1] rope used for
ritual purification in the
Shinto religion. "
"A space bound by
shimenawa often indicates a
sacred or pure space, such as that of a
Shinto shrine."
"
Shimenawa are believed to act as a
ward against
evil spirits"
"They are also used around
yorishiro (objects capable of attracting spirits, hence inhabited by spirits). These notably include certain trees, in which case the inhabiting spirits are called
kodama, and cutting down these trees is thought to bring misfortune."
"A variation of the
shimenawa is used in
sumo wrestling by
yokozuna (grand champions) during their entrance ceremonies to denote their rank. This is because the
yokozuna is seen as a living
yorishiro (formally
shintai), and as such is inhabited by a spirit."
So If you have evidence to the contrary, please provide it.
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We can - and I expect we do - add Fairy elements for Jumbao by the Ability and the Pokedex entries, and the typing would be justificated. That, and the movepool has quite good support options around it's benevolent nature.
While I don't dislike your idea of the contrast between the prevo and Jumbao, it is our big guy's movepool the thing that restricts us a bit. For a Grass/Ghost mon, I'd say Forest Curse is a must, but Jumbao doesn't have access to it.
I think we are limited to Grass/Fairy and Grass
I just don't really see the movepool as that big of a deal lore wise. Flavor abilities are seen every time you go to select an ability and can give a bit of lore to the pokemon and while not that easy to just stumble across, pokedex entries are a good way to give a concentrated dose of lore in a succinct manner. Move pools are neither succinct nor are they commonly seen. Most of these flavor moves will be relegated to the "usually useless moves" section on Showdown. How often do you go there to get a sense of flavor about the pokemon? But that's just my opinion.
Edit: If you're willing to stretch a bit,
Shore Up is a move that only ghost types know even though it is not a ghost move. Over half of all ghost pokemon learn
Psych Up and nearly half learn
Magic Coat.
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You still have not explained how we can justify Jumbao's lack of Ghost type moves (sans Shadow Ball) and flavor moves for ghost types
I think Jumbao's lack of ghost moves(besides shadow ball) could be easily explained as it not being able to perform those moves as the spirit that once inhabited it has left. The real problem IMO is the lack of ghost moves for the prevo. Maybe the justification could be that the spirit is tapping into pre-Jumbao's strength for attacks rather than its own and so it has a hard time using ghost type attacks. Ghost flavor could also be added in the form of a flavor ability for pre-Jumbao such as cursed body. I'll admit that the type doesn't fit well movepool-wise though.
Edit: Or maybe pre-Jumbao still has a sense of honor even though it is possessed by an evil spirit and it resists using those moves or something like that.
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I'll do one more post to try to convince people to at least consider
Grass/Ghost.
I think that this typing would allow for a more interesting design for the preevolution. If we keep Grass/Fairy, the prevo will probably end up as basically just a small version of Jumbao. Not that that's necessarily a bad thing, it's just not that interesting IMO.
The typing would fit really well with the lore behind Jumbao's design. The root rope thing around its waist is a "Shimenawa" which, according to the artist is "hung around sacred objects like trees or rocks and wards off evil spirits that can inhabit them". So the lore given for Jumbao's design specifically mentions that spirits can inhabit trees and that the Shimenawa is placed around it to ward them off. So the prevo could be inhabited by an evil spirit and that is why the Shimenawa was placed around it in the first place.
And lastly, I think the ghost typing would reinforce the fairy typing of Jumbao. Jumbao's design doesn't really scream "fairy" to me. Fairy seems like a sort of opposite to the ghost type. Obviously there's the whole Clefable - Gengar connection, but beside that, ghosts feel more malevolent, sinister, evil, and corrupt while fairies are more carefree, malevolent, good, and pure. I think a good way to draw those characteristics out of Jumbao would be to contrast it with its prevo. The fairy typing would make more sense because the evolution from pre-Jumbao to Jumbao would represent a purification and transition from an evil spirit to something that is good natured.
TL;DR: Grass/Ghost is more interesting, it fits the lore very well, and it reinforces the fairy typing of Jumbao.