A Lowepro Prorunner 300AW swallows into the main compartment, based on my MF kit:
  • RB67 Pro S with attached 180mm, 120 back, wlf
  • 90mm
  • Spare back
  • L-grip
  • Filters
  • Film
  • Cable release and odds'n'sods

Plus two from three..
  • 360mm
  • Polaroid back and packfilm
  • Sekonic L-758

And that's about as much as I want to carry weightwise, along with the tripod on the external clip. If you want more, it has attachment points for the sliplock system. And it's small enough to fly European budget carry-on.

But, there is no magic bag.. and one bag will never be enough