I think affecting I-frames is the better choice. The changes you propose would make rolling primarily a method of rolling away from attacks instead of also being a viable method of rolling through attacks. Being able to roll through attacks results in better and more entertaining gameplay based around timing. If better rolling away from attacks were the primary benefit of increasing agility then it would place even more emphasis on baiting attacks, backing up/rolling away and attacking during recovery. Rolling through adds to the variety of gameplay by being able to time rolls to avoid attacks and still stay in melee range.
I do agree that a visual way of indicating this would be good but I really can't think of a very good one.
You can roll through attacks with, say, 12 iframes (between medium and fast roll in DaS1) perfectly fine. Finished DaS1, rolling through everything with medium roll (11 iframes) and it felt perfectly fine.
Having a roll that completes faster (less frames in total, so the iframes to total frames ratio is increased, same as if you increased the iframe count) lets a player be more aggressive, actually, since you finish the roll sooner and have a larger window to capitalize on the enemy's vulnerability. It would also let you roll more "reactively" - you could wait longer before starting the roll, and still roll through the attack. Finally, reduced stamina cost would improve aggressive potential as well, as you can spend more of it on attacking, and need to leave less up in case of emergency.
On the other hand, a character in heavy armor/without ADP would have to time their rolls and manage their stamina more carefully. They'd still get the same iframes, but would need to start the roll earlier, and be punished more for mistiming it and getting caught at the end of the animation.
I think you either do this or go back to DaS1's system, maybe with more breakpoints to stop players from optimizing too much? I think it's very important to have a visual indicator of how a roll's defensive properties change. 8 to 15 frames is very little, you only perceive that intuitively after practicing, so if it changes, everything feels off. Playing DaS2 with 85 Agility (8 iframes in a roll) is a very awkward experience, as
Mikayel can attest.
