I favour keeping Barges in a fairly traditional role of mobile storage. I'd like to see their speed increase to what it was in 4.85 in exchange for moderate limitations on where they can dock.
As far as armour is concerned; I've not spent a lot of time shooting a Barge or being shot in one so I make no comment. If one were to play around with cargo capacity, plus or minus 10,000 (ie. 30,000 to 50,000) is acceptable. If the cargo capacity decreases, the speed of the ship absolutely needs to increase.
Earlier in the thread, the concept of an all-in-one jump carrier-style ship has been suggested; while that's a really nice idea, I don't think the current Barge model really looks the part for something like that.
Making Barges available (by request, even if not like standard ship purchases) is a necessity. They are a seriously steep investment on the player market at the current time; the value of which only represents their rarity, not their utility.