Account related
Riot generally wants people to open support tickets for this, as it's usually private to the individual user.
New player experience
I completely agree, and for some reason Riot seems to think the Tops & Tricks board is acceptable for new players (spoiler: it's not). I have a strong suspicion that Riot doesn't particularly value the onboarding experience since so many people already play League of Legends, and it'd almost be a waste of resources to pursue it at this point.
Feedback on skins/chromas
This generally goes in Skins & Champion Concepts or Miscellaneous, and I think it should stay that way. I think it's dumb that champion concepts are in the same board as skin discussion, but that's neither here nor there.
Mastery System/Ranked System/client features
I would really love to see a PBE.net board or something similar to allow players to discuss these sorts of topics. There really isn't anywhere on the platform to do so other than Miscellaneous, and since Miscellaneous is basically Off Topic it's really hard to form a community around any particular initiative.