tl;dr: improve class fantasy and increase rewards without upsetting game balance.
Disclaimer: I’m a fan of both diablo-likes and soul-likes, but tend to prefer the former mostly because of the deeper customization, rich class fantasy and hardcore mode. Ironically, my favorite soulslike is Sekiro.
Support pure ranged characters without trivializing the encounters.
tl;dr: two-handed bows / special class of staves have basic attacks which only do damage from the rear or on staggered targets, but reward lots of focus.
Currently, having to melee to gain focus to occasionally use a ranged attack feels bad for players who want to play a ranged character. Creating a new class of bow (longbow) and staff and giving them a basic, quick ranged attack without upsetting the game balance would be quite simple to do. If said attacks are deflected from the front/sides if enemies have poise (and do not damage said poise), it will force the player to do the melee equivalent of fishing for backstabs (but with a more lenient angle, and without a special animation). These attacks should generate quite a lot of focus which could then be used on runic attacks which would work as they do now. When attacking from the front/sides, produce an appropriate audiovisual response that lets players know the attack was deflected off of the armor / shield / weapon / carapace, what have you.
You could keep short bows as they are now (offhand weapons) to maintain the hybrid melee/ranged combat which some players may enjoy, and add bucklers to the side of the longbow’s shaft so they can be used to parry and thus do even frontal damage with the basic attacks, becoming viable but challenging primary weapons.
The only problem is that focus with a longbow / special staff equipped would need to decay quickly over time after a certain grace period, to prevent players from banking focus on easy mobs and then spamming fireballs on a tough mob’s face from a safe distance. I think this would be reasonable.
I think there are several creative ways to make pure ranged characters work without having to make significant AI changes to every mob in the game. This is just one of them.
Allow player to experience his desired role from the start.
tl;dr: add souls-like soft classes with starter equipment, or choose starter weapon / basic rune at char creation.
Currently a player may go in with the expectation of playing a certain role, but be unable to do so for potentially a long time due to the random loot. This feels terrible.
As in the souls games, adding small attribute buffs to the soft classes to teach new players which attributes they will most likely want to focus on could be a good thing. Maybe a tutorial popup explaining how the chosen starter weapon(s) benefits from attribute scaling could accomplish the same thing and be much easier to implement.
Feed the dopamine receptors and fashionistas.
This is more of a 1.0 / post-launch suggestion which no doubt you’ve already considered, but increasing the amount of loot without upsetting the game balance is possible through cosmetic drops, such as reusable weapon/armor illusions (transmog).