I assume you are talking about building a kayak that would be easy to learn how to roll in and not a "rolling kayak" for fancy Greenland rolls" correct?

In the beginning I think one of the best assets in learning to roll is good thigh bracing. Many builders often feel that they are locked in well because they can push the kayak around a bit when paddling. The problem is: when you roll you have to be able to twist the boat up from a side ways position and often paddlers legs slip out of position. If you are in your kayak on the lawn, roll on to your side - that's the position you are in when attempting to roll up. You thighs end up pushing against the thigh bracing sideways as you try to twist up and twist the kayak up too. That's why good bracing has a downward curve to lock you in. So if you try to learn to roll, be conscious of that because I think it's the leading reason many people fail. One trick you can do is back your foot braces in a notch so you don't accidently slip out of your bracing. Once you get it down you can cheat all over the place and you'll be able to roll any boat even with crummy bracing.

Having a low back deck makes the lay-back roll easier but many people learn the hip snap type of roll and generally the difference between hulls is not that great. But if your legs slip out of the bracing, you'll never roll. One of the reasons I love the Ocean Cockpit is because I have a block in the center and you can never slip out of position. The kayaking world is pretty much a large cockpit world and you can lock yourself in well with good bracing.

Ironically, I sometimes find a large bulbous, fat 24" wide kayak, very easy to roll since it's so round like a big pipe. A hard chine version of the same boat is noticeably harder to roll. My first 24" Chesapeake Cape Charles is a bit stubborn for beginners.