Hmm.. considering how I'm a huge fan of the original four or so optional bosses in KH1, I think I might join. Don't expect me to be all too active though, as I'm not into fanclubs as much anymore.
My most favorite non-Sephiroth KH boss is Kurt Ziza by far, though Ice Titan was pretty cool, back in KH1. While this made the battle time consuming in KH1, I felt it added variety and made him seem more Zelda-esque, as in not being entirely capable of directly damaging him. The only way to damage him for most of the battle was deflecting his ice missiles back at him, then hit him while he was down for a few. Fire seemed to suck against him for some reason though and I found out the hard way about using Aero against him, when I was doing a low level run (Expert mode) and all. He actually beat me because of it. I ended up leveling up some more, then fighting him again.
By the way, he seems kind of lame in Birth By Sleep. If only they would of made the legs thing only another way to knock him down, instead of a way to directly attack his HP bar. That would be cool if you had a HP bar that kept coming back and it was to knock him down, then hit him in the face when he's down, ala KH1.
I thought Phantom was cool to. I just wish I could of figured out the colors without looking at a FAQ on Gamefaqs. Too bad, he's more or less limited in his attack arsenal. Also, KH1 Sephiroth > KH2 Sephiroth imo.
Anyhow, if I can somehow get a PSP, I'd love to beat its optional bosses, including the ones in Mirage Arena. Iron Prisoner 4 looks beastly. Honestly, after spoiling myself on the battle, he seems like one mothafcuker you don't want to mess with. One thing I love about him is in one video I saw, he just went berserk right as the person was almost dead and ended up knocking them out. Luckily, it was co-op, so they got revived, but still. He went fcuking berserk! Berserk I tell you. It looks like BBS has finally brought back the optional boss to the KH series and for real this time, not just another Sephiroth battle, something in FM, or a recolored Heartless (boss). The ones I saw in the Jap version of BBS alone seem pretty menacing.