I think they could be built a lot better than they are. They are kinda like pontoon boats, whereas the manufacturers build them in the cheapest configuration possible, because the consumer doesn't seem to care.
As an example:
1) They could drop the engine down into the bottom of the boat and run the fan off of an extended belt - this would lower the center of gravity and make the boat safer (I have seen these things flip over easily when hitting a stump, even at slow speed).
This has been available from Century Drive Units without much interest for many years. The unit weighs more than a typical belt unit, cost more, and usually doesn't hold up as well.
2) High-thrust counter-rotating blades could be used for increased thrust at lower rpms. Most of the noise an airboat makes is from the blade tips breaking the sound barrier - if you can keep blade tips below Mach 1, they will be much quieter and mufflers would actually help on the engines then.
Most blades in production today are a wide designed with both thrust and sound being the main objective. This added with increased reduction unit ratios up to 2.88:1 have made huge leaps in both performance and sound. Counter-rotating reduction units have also been around for many years but held back in popularity due to cost.
3) Add PTFE (sp?) to the hull bottom so less engine speed is required when running across grass, pavement, etc.
Quick-slick, Frog-spit, and others are used with reasonable success in Florida where grass is the worst surface. Even with grass, these coatings must be re-applied ever year or so. In Texas, most rivers are gravel, rock, or sand which would strip these surfaces within minutes of dry running. Oysters are even worse. 3/8" to 1/2" UHMP "poly" has been the product of choice for a reason. Silicon impregnated UMHP is also available depending on what you want to pay.
4) This is mostly wishful thinking, but a sprinkler bar could be added to the bow to wet the grass, and the boat would slide much easier over it...