Touhou Adventure

Nope, no fan-games to link here, sorry to disappoint on that front, but since we're talking about two games at once, it seemed fitting to put it into the games section.

So let's get right down to business, the characters and their themes:

Sonic the Hedgehog - Reimu Hakurei, Final Boss Theme
Egg Viper - Konngara/Sariel

Miles "Tails" Prower - Marisa Kirisame, Final Boss Theme
Egg Walker - Mecha Marisa

Knuckles the Echidna - Youmu Konpaku, Final Boss Theme
Chaos 6 - Saigyou Ayakashi

Amy Rose - Toyasotomimi no Miko, Final Boss Theme
Zero - Fujiwara no Mokou

Big the Cat - Alice Margatroid, Final Boss Theme
Chaos 6 - Shinki (Might change?)

E-102 Gamma - Reisen Udongein Inaba, Final Boss Theme
E-101 Beta - Eirin Yagokoro

Super Sonic - EX Reimu, Round 1/Ending Theme

Round 2/Final Boss
Perfect Chaos - Unleashed Yukari


Now, to the gameplay of each character.

Sonic/Reimu is fairly straight forward. Go through the stages, beat the boss at the end. She also has the most story out of all the characters, good good.

Tails/Marisa is another one that's straightforward, they go through stages as Sonic/Reimu do, but they race against an NPC along the way to the end. Since Marisa's boss is Mecha Marisa, each stage can be a race against Mecha Marisa [either to prove which is the 'best' or to prevent Mecha Marisa from doing something].

Knuckles/Youmu is where the gameplay is switched up a bit, you're no longer progressing to the end of the stage, you're searching for pieces of the Master Emerald/Parts of Yuyuko's Spirit. The Master Emerald was destroyed by Chaos, and Yuyuko was destroyed by the Saigyou Ayakashi. Which is their respective end bosses.

Amy/Miko is another character where you progress through the stage, but now the goal is to run from an enemy that cannot be defeated. Amy is doing this to protect herself and a bird. So Miko can be doing this to protect herself and Tewi (And be prepared for that Tewi thing because I'm gonna blow your frikkin' mind.)

Big the Cat goes LOL FISHING FOR A FROG in a Sonic Game. Take out the fishing entirely and have Alice going through stages to find her lost doll, Shanghai.

E-102 Gamma is arguably one of the more interesting characters. He only appears in Sonic Adventure [as far as I know], he's created by Robotnik. So, you can already tell how well things are working out for him. E-102 Gamma's gameplay consists of shooting your way to your objective. Reisen is therefore a logical choice.

Super Sonic is of course, where the entire story wraps up. This one consists of mostly story and cutscenes. And ends with getting the Chaos Emeralds, going Super Sonic, and beating Perfect Chaos. So, the same principle for EX Reimu applies.

I know it's very unlikely that anyone has read this yet, and that this is in fact a double post, but I'm doing it to keep everything nicely organized.
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So I'm planning that, in order to unlock EX Reimu's Chapter, you have to do a few things first.

The most obvious is, finish all the character's stories, that's what you have to do to unlock Super Sonic's story. Now, to add onto that. I'm thinking that there should be something else that you have to do.

Namely. Defeat a Secret/Extra Boss for each character. And in order to do that, you first need all of that character's upgrades. [Light Speed Dash, Light Speed Attack, etc]. Doing so obtains a Chaos Emerald or whatever is chosen for Touhou Adventure.

I'm still deciding on some Extra Bosses [hell, we don't even have the entire 'main cast' yet], so look forward to that as well.

For now, I'll give you a slight tease, note how I said "every character gets a secret boss". Make sure you don't skip any characters when counting up at the first post.

Ah, how about Kanako for final boss? She could be riding a giant snake or something like that.

Or an "unleashed" form of Yukari?

I'm actually not sure yet. Ah, the round two boss battle music is temporary by the way, until we figure out who to choose.

I'm thinking that an 'unleashed' form of Yukari would be fitting, perhaps something happens to her, causing her powers to run out of control, and perhaps herself as well?

What ever the chosen "Chaos emeralds" are over activate her powers?

Yeah, Yukari could start off just havin' fun and then she happens across one of the 'Chaos Emeralds' and then oh shit son, it gets real.

What are the "chaos emeralds" anyway?

I was actually trying to find their official name but I can't.

Basically, when you lose but pick continue, Touhou gives you these "F" Squares. And they serve as a full power up.

They're consumed on use and then serve no other purpose.

So I was thinking there could be seven "special" variations that are not consumed, and then a giant "F Square" serving as the Master Emerald.

They could be gem-shaped, rather than actual F-Squares.

Gem shaped F's huh?