Game features, characters, settings, story, anything. However, avoid something as fruitless as "more characters."
Touhou 11 went underground, Touhou 12 went overskies. I'd like to see Touhou go underwater.
The story could be something like the main characters get word that the shinigami haven't been able to cross the Sanzu river. The main characters start out at the 中有の道(Chuuu no Michi, look up Perfect Memento in Strict Sense). The first boss could be an employee from hell (running one of the small shops there). They start to cross the river from 此岸(Shigan). Second boss could be a new shinigami (or Komachi, saying everything's fine I'm doing my work, ZUN style). They find a mangrove forest in the middle of the Sanzu that doesn't seem to be effected by the river's nature, which stops the characters from dying (when someone crosses the river, they would usually die). Third boss could be a youkai wearing some kind of armor made of bones which washed up onto the forest, representing those who drowned in the river, because they couldn't cross it. They dive into a whirlpool in the middle of the forest to search for the origin of this magical mishap. The fourth boss could be a Siren (water demons who sing people to death). The characters proceed to the core of a whirlpool that's been messing with the river's magic. The last boss could be an 天魚(Amana), which could be in the form of a mermaid. Beating her would stop the whirlpool and the torrent of magic that's messing with the river. EX Boss? Perhaps a 水龍(Suiryuu).
鏡月天魚(Kagamitsuki Amana), the last boss. She would be a mermaid with red wavy hair and a fish tail that fades from white to red towards the tip. She would have a trident. Some of her spellcard names may include terms like leviathan and undine.
The only reason I came up with this idea is because I think mermaids (人魚) are the only creatures ZUN hasn't nearly touched yet.
As for game features, I think it would be cool if ZUN added shots that came from underneath the character. For instance, this idea may have underwater geysers emerge from visible holes in the game background.
The UFOs from 12 and the option antennas from 11 were unique additions that fit with the setting of each game. This idea could have a bubble kekkai(shield) that needs to be regenerated by gathering air from fairies and ghosts.
So, what do you want in the next official Touhou?
The weird thing about this thread is, if ZUN read it, he wouldn't do any of it. lol