I just got done with my early access run and I wanted to offer my feedback.
I enjoy playing around with illusions when I play 5e and I think giving players a choice in what they make - kind of like what you're doing with Disguise Self - would go a long way towards making an illusionist both fun and viable. Here are just a few suggestions, which may or may not fit.
The player conjures up a terrifying visage, forcing enemies who see it to make a saving throw. If they fail, then they spend their remaining movement fleeing in the opposite direction. Alternatively, it could give them the feared status.
The player creates an illusion of a valuable jewel lying on the ground. Ideally an npc's reaction would be based on their personality, but since this is a video game they'd probably have to succeed on a saving throw or they'd walk over to pick up the object... setting them up to get punted off a cliff, locked in a cage, etc.
Something distracting is created, such as a doomsayer trying to preach about the end of times. While enemies are distracted, player characters have advantage on stealth rolls.
Here's some more general recommendations/feedback.
Please stick to 5e's ruleset as closely as possible. I know some changes are necessary since this is a video game(and if I'm being honest 5e's ranger class needs a buff), but it's a pretty good system for the most part and I know a lot of people really want to feel like they're playing D&D 5e in video game form. I feel like this will make balancing the game easier as well.
It'd be cool to see dice rolls during combat. Maybe as an optional feature, since I'm sure just as many players don't want to see them.
If you're not already planning on it, you might consider adding some subclasses not in the PHB. Personally I'm a big fan of Swashbucklers, but maybe you could do polls to figure out what people want most.
Would it be possible to change how surfaces work or is that baked into the Divinity engine? I'm thinking fire would be less annoying(and more true to D&D) if it was slow to spread, but would also spread to wooden objects such as crates and walkways. I feel like other surfaces probably shouldn't spread at all.
A way to tell the whole party to do things would be a nice QoL change. Sneaking in particular comes to mind.
I thought resurrection scrolls seemed a bit rare. It'd also be cool if you could have someone strong(such as Lae'zel) carry her dead companions body to camp to be stored until resurrected.
Personally I think allowing two short rests would be better than just one. It felt like I was relying on long rests a lot simply to renew my spells.
Food almost makes healing too easy. Since I played without Shadowheart, I figured I'd have to invest some money into healing potions.