What does it take to become a BIAB "expert"? I guess the issue I have there is that you could probably just as easily direct people to other forums and sources for just about any topic that is covered here. There are probably more "expert" types hanging out over at the AHA forum than here.
I still think BIAB is a minor variant of typical all grain brewing (granted one with a high level of regional acceptance). If I put a bag in my mashtun to aid in cleanup, but otherwise run it as a fly sparged, three vessel system, it's completely arguable that it's BIAB. I'd just be using it either as my separation medium or to augment some other medium like a false bottom. I could just as easily argue that RIMS/HERMs style systems are far more unique as an AG variation than BIAB is.
1 tier, 2 tier, 3 tier
fly sparge/batch sparge/no sparge
1 pump, 2 pump, no pump
false bottom, braid, manifold, polishing pads, (voile bag)
1 vessel, 2 vessel, 3 vessel, HL on demand
Mash heat: eRIMS, dfRIMS, HERMS, Direct Fire, None.
BIAB, depending on who you ask, would be constructed in definition with a certain combination of attributes above.. something like 1 tier, no sparge, voile bag, 1 vessel, Direct Fire (or none).
All the other ways you could combine them would be left to the AG forum.