haha - my mind went to that idea of a "ring" for the subforum as well. (And the ritualized leg lifting stuff in the sumo ring makes me think of your recent thread about Tibetan debate...)
I like the idea and feeling of "koohii lounge" and "koohii break", but share Nest0r's view (if I understood correctly) that the koohii connection seems a bit tenuous.
If Fabrice wants to use alphabet, maybe an English word would be better? (I actually didn't even realize it was コーヒー for a while. duh.)
I don't know the history of the koohii name, but it doesn't seem particularly descriptive or reinforced in any way by the current name, concept, purpose, image, or logo of the site. Since some forum newcomers are unaware of the main RevTK site (at kanji.koohii.com) and the forum is currently at forum.koohii.com, it had crossed my mind that a shared name for both which somehow conveyed the RevTK connection might be helpful. (or whatever RevTK becomes)
I'm not suggesting the domain name ought to be changed (no idea what technological or admin hassles that would involve), just that if you choose to name the subforum "koohii lounge" you might want to consider how "koohii" fits with the overall concept or "brand" now or in the future.