I go along with many others above who have stated that manufacturing quality, attention to detail, and quality of components used should be the major focus for the "top of the line" Heartland unit. Landmarks look great on the outside, but durability and reliability can be improved by paying more attention to the stuff that is not so visible.
Example - I was at my dealers today for some warranty work. I asked them to look at the support under the fresh water tank. When the tank is partially full, it pushes down hard on the coroplast belly covering such that it stretches and doesn't seal properly. water gets in when it rains and soaks the insulation. The insulation under the tank is worthless as it is so compressed. The dealer could do nothing to help here - it's a design deficiency. So make the tanks/supports such that tanks are properly supported and all of the tank inventory can be used. The wiring, plumbing and ducting in the belly area looked haphazard.
Other examples,
Use brass not plastic fittings on PEX.
Better closet door catches.
Make the underbelly air and water tight.
Features,
Improve the suspension - why not take a leap and put in adjustable air suspension? This is now common on many commercial vehicles, I even saw it on logging trucks in Alaska and BC this past summer.
Go for disc brakes, many other trailers use them, it's a simple change.
Overall though - limit the gimmicks, really push the quality, add good running gear.