A major problem is pressure. As the fuel heats up and boils off the pressure increases and the tanks have to bleed it off or they'll burst eventually. Also the engine design for cryogenic stages assume a liquid fuel and moving enough gaseous fuel and oxidizer isn't in the current designs.

That's a good question. A big bag of fuel hanging outside is a fine place to store it if you don't care about temperature or pressure. You would need some kind of mechanical means to squish the bag to get the fuel out though, so maybe an accordion with a motorized retractor?

I think you are underestimating expansion ratios (ratio of volume of liquid and gas at reasonable pressure). The ratio for water vapour at sea level is ~1000x. The "large bag" might end up being so large that it's weight is a significant fraction of the weight of the fuel it contains.

Even if you brought up a bag large enough to contain gaseous fuel and oxydizer, before you could actuall use the fuel you'd have to compress it again.