I'm an aircraft maintenance engineer for an airline in Australia, fixed wing aircraft sadly, but my coworkers and I have been talking about this for a while. (On and off, in between discussions on wind turbines, electric cars, etc)
I think that with a modern composite structure, a good strong modern fabric (racing yachts use woven kevlar, don't they?) outer hull. Efficient diesel generators, and several (six or seven) strong electric-driven propellers or ducted fans that pivot to provide maneuverability... Then it could work. I favour the hybrid airship model, with at least some lift being aerodynamic, where one of the guys at work favours pure lighter-than-air. In civilian applications, the airship would be flying from a modern airport anyway, so issues with ground crew numbers and such aren't that bad. We have three guys seeing in one aircraft, two to push it back. A hybrid airship shouldn't be much different.
With gimbaled pivoting electric motors you would avoid the issues of low speed maneuverability that the Flying Cloud has. And also you could probably fly these in to airports after noise curfews are in place for other aircraft. 50 tons of cargo in at 2am without a sound! (almost haha).
A modern airship ought to be quicker than the classic airships. Even at 250km/h, it would take eleven or twelve hours to get from Perth to Sydney - longer than an airliner's 5 hours, but not too bad. And the alternative is three or four days by bus/truck or train! To cross the pacific (refueling at Haiwai'i) would take two days, I think. (I can't remember the numbers at the moment). All up it may take four days of flying and airport transfers to reach your destination (Perth to Toronto in this case) but I'd much rather spend a few days in a comfortable chair, watching movies or reading a book or something, being nicely refreshed when arriving at the next airport and not having as severe jetlag - than spending 36 hours in a cramped cattle-class seat, getting no sleep and being stressed out at a foreign airport, taking a week to get over the jetlag! haha.
I think it's not a matter of engineering, but finance and public opinion. No doubt if someone tries to introduce an airship on a passenger (or freight) route, someone else will jump up and down and cite the Hindenberg disaster.
Wow, that was alot more convoluted than I thought it'd be. Oh well, bring on the new age of the airships! I'd love to be an engineer on board one. Much better than being stuck on the ground.
*Edit - hope that's ok.
It occurred to me today that a major advantage that a cargo airship would have over conventional air cargo is that since the cargo bay probably wouldn't be pressurised, and so there is much more freedom in the shape it could take. Which means instead of using aircraft cargo boxes (wikipedia tells me they're called "unit load devices"), you could use intermodal containers taken directly from a truck or wet ship or whatever. You'd still be limited to a small number of them (the max weight appears to be just over 30 metric ton each (!), more than I thought it would be), but a large ship might carry ten or twelve (say, 360,000m^3 of helium for the cargo alone, plus airframe, engine, fuel, etc. Quite a large airship!), and would still be quicker and much more flexible than a train.
it'd be an impressive sight!