One thing I'm considering is a master puppeteer for the whole Shimon group. For the moment they're data-gathering, sincerely, and sincerely liking/respecting the kids they're paired with. When the time is right they'll be made to do things they don't want to do. That way they can indeed be the enemy and not give that vibe to even eagle-eyed Reborn. This setup would be turned into a set of mirror-nemesis fights, which will be hard to plan out, but wonderful if executed well.

Jinchuu arc in RK was scary in this way, a completely different way from Shishio's Kyoto arc. Shishio just did his take-over-his-known-world thing, but Kenshin came to stop it. Enishi, on the other hand, specifically targeted Kenshin, preparing for all his known techniques and attacking all he holds dear, so Kenshin was unnerved from the inside and outside, it took a LOT to get him back.

I'm suspecting the guy in the fish tank knows more than he's telling, but I'm just paranoid that way.