Funny thing but I was just thinking about this yesterday!
My dream invention would be a gravel cleaning system similar to an undergravel filter. The system would have a tank attached underneath the fish tank with a connection to the two (somehow). You would be able to turn it on and it would suck out all of the built up crap.
I would also add another members idea of having a built in water system in the wall so all I would have to do it turn the water on to fill thr tank back up.
It would also be nice if I had an something to measure the all water parameters that would give me the results in digital form.