(at the risk of invoking Jobs's line about, "If Henry Ford had asked people what they wanted, they would've said, 'A faster horse.'"...)
What should be added? Which hardware feature? What kind of upgrade? Should it be an OS-only upgrade? Should it break compatibility with older apps? Should it be a dual-lens camera? What if it changed from Lightning to USB-C?
You say you want a reason to upgrade. What would push you over the edge and convince you to buy a new iPad?
What is interesting about the iPad is that part of its beauty is the simplicity and the way it works with the same OS as one of the most successful consumer products in history. I know enthusiasts won't be happy until it runs a touchscreen version of OSX, however, it will not longer be the iPad at that point.
The real problem is that the old versions already do what most people want them to do. It is just a matter of knowing what the upgrade path is now that there is saturation in the market versus when it was introduced 7 years ago and no one had one. Obviously, it doesn't have the benefit of being in the pocket all the time, so camera upgrades, which help drive new iPhone sells, is a much less powerful incentive to upgrade.
I think people are misinterpreting market saturation, and new competition, as the product "dying". No, the product, at least in units, is not dying: iPad sold 13.1 million units, Mac sold 5.3 million. Yes, the margin is lower, so they don't always make as much money on the iPad, but that has nothing to do with popularity.
Unlike last year, they did not introduce a new iPad with a different form in the fourth quarter, so I would expect their numbers to be down year over year.
Anyway, we all have ideas how they could make the iPad better for people that want to replace their PCs with it. However, that may not be the route to get the mainstream audience to upgrade. The fact is that the iPad people currently own may already do everything they want from an iPad. Some people prefer a butcher knife and a chef knife over a swiss army knife when the task calls for it.
I am definitely interested in what they will come up with to help drive sells, but unless it is a new product category, they will continue to compete with the iPad people are already using at home.