Realistically, I would choose Strength and Toughness, and I somewhat like them as concepts, however, AoS avoids many of the problems of 40k by preventing instances of a hopeless fight. Everything can kill anything in AoS, which from a fun perspective, if far more enjoyable than watching my Kroot beat on a Bloodthirster hopelessly.
Now I really didn't play much of 8th Edition 40k, so most of my experience is from 4th through 7th, and in those older versions, if you didn't have the tool for the job, you couldn't win a fight. It also had a real issue with 'Deathstars' which AoS avoids completely, and that could be compounded by being impossible to kill.
Personally, as far as AoS is concerned, I would just increase the wounds on monsters significantly, and with the appropriate balancing, especially with Hero monsters. I could also see tougher units have a -1 to wound them. So for example, if I was designing the Mega-Gargants, I'd give them a -1 to wound for attacks against them that have 1 damage.
On another note, many units in AoS do far more damage proportional to the target's durability than they really should be doing. Most units sit on a 4+ or 5+ save as their only defense, while that unit of Witchelves is about to nuke you three times over. On the flipside, you end up with units with 4+ DPRs.