Edit* oh almost forgot, Solasta ui looks like a mobile game.
Solasta's UI asthetic is usualy the main and only criticism that comes up... I don't think anyone suggest imitating the look of the thing - it's just the only example of a better functionality of 5e system.
Really anything else would do. While Solasta's decision to split Main and Bonus actions entirely, is interesting, I don't think Larian needs to go that far. Larian seemed to take a lot of lessons from Dragon Age: Origins (be it UI or control system) but they need to understand two things:
1) spell heavy classes will have too many spells at their disposal that will rotate in and out on regular basis - it will be impossible to fit them on the hotbar, and it's too much to manage after every level up long rest. Dragon Age was shallow and streamlined even compared to 5e. While organizing skills was also frustrating there to me, there were fewer spells total, not many useful items, and really no big consideration to make when stuffing spells on your hotbar.
2) Dragon Age and Divinity:OS1&2 doesn't use resources for most abilities - there is no spell level, action/bonus action to worry about. Only cool downs. Where an ability is in DA and D:OS1&2 doesn't matter, as all we need to be able to do is find it. In DnD we also need to worry about which resource it will consume, what other spells we might be prevented from casting if we use this spell now aka. how much of this resource we have, and which action goes with the spell. It is a more detailed system, and UI doesn't support it properly.
