I think they should add a lot of dirt in low areas in New Orleans.
Let's look at economic of it. Suppose you had a lot which was 20 meters by 40 meters. And you wanted to add enough dirt to make the lot 10 meters higher, that's 800 cubic meters. You could probably easily buy dirt delivered at around $5 a cubic meter, so the dirt would cost $4000. Now you would need A retaining wall and this is would cost a lot more than $4000 dollars, but the your road is also being raised and everyone in your area is adding this dirt, so you don't need a retaining wall. Now if you living in low areas would it be worth $4000 to have your house 10 meters higher- would be worth being higher than sea level? I think in flood insurance cost alone it would pay for it in a couple decades.
What would happen if dump dirt in all the areas which currently flooded, so it was higher than current water level. It wouldn't spread to other parts of city, instead it would flow back out the levee which is still broken. And of course if the dirt had already been there, the city would not have been flooded in the first place and billions of dollars in damages would not have occurred.

But this storm was freak event and it probably wouldn't worth it if the sole reason was to guard against this type of event. But normal flooding is probably pretty common and even evacation for a future hurriances which even "missed" or lessen in intensity is a expense ordeal. Additionally right now there is an opportunity with this disaster- all the house flood are basically wrecked and would require much effort to repair. You instead could simply bury them. Additional you can use the water to move the dirt in a more economically way- barges can move much more dirt than dump trucks. And since you moving huge quantity of dirt, instead of create flat land, you might as well make some gently rolling hills. So after you done the lowest areas could 10' or 20' above sea level and highest might as much as 100' above sea level. So I'm talking about a massive amount of dirt. Now, some areas are historic, and you might just leave those areas at the present level [below sea level] but if half or more of the total area is build up, it would easier to protect these smaller areas.
Anyways, using big land moving machines, using dreg material which bought to area with barges, in other words not just by tiny dump truck load requiring millions of them, but a mass scale operations, if managed well you could cut the costs so you looking at something like a 10 billion project. The dirt part is pretty cheap, the whole project, including subways, flood control system that can actually drain into the ocean instead requiring to be pumped, underground electrical power and cables, and all the bells and whistles might end up around 50 billion dollars.