Back when "Taiki Bansei" came out, there was some of talk of Takui Nakajima eventually filling the songwriting/production void left by Tsunku's departure. That never materialized, but a guy who has flown under the radar the last four years is Hoshibe Sho. Go check out his songwriting credits at his H!P Wiki page. This dude is good, has incredible range, is pretty young at 34 and, unlike Takui, does not appear to have professional aspirations beyond songwriting. I don't think we'll ever see one person take on the amount of responsibility that Tsunku once carried in H!P, but I hope they do whatever is necessary to keep this guy on staff. Increase his rate; drugs and hookers; fake his death and lock him in the basement. Whatever it takes, because this dude is killing it.
Full disclosure, this thread was really just an excuse for me to fawn over Hoshibe Sho, but feel free to talk about any other H!P songwriters you like.