An origin story where he isn't an idiot following whispers in the night but someone who has been trained to be his fathers successor, had court intrigue and plots against his father something he has been aware of his whole life and trained to deal with, but then gets progressively, carefully and subtly enmeshed by Mizora/Zariel over a period of time. Whichever of them sowing discord between father and son, making Wyll question his fathers decisions more etc, culminating with some trap event that results in his pacting when his father is away. The pair (or just one) wanting a route in to influencing the inner circles of Baldurs Gate politics (or at least trying to affect his fathers decisions) would be far more interesting and doesn't need Tiamat's dragon cultists. Something playing on his inexperience, but done more cleverly, and centre it around city intrigues and factions. I don't think he needed a dramatic dragon head appearance to make his story better, just more intelligent content and its not as if any of it gets shown so shouldn't be that hard to do. It would also go further in explaining why his more experienced father booted him out of the city. Not just because he has a demon by his side but because said demon was trying to influence things in the city. The events with Karlach should have Wyll questioning the pact entirely, discussing it with Tav and realising he was fooled, and that his father was right to kick him out of the city all along (which he should have resented somewhat in my scenario, after all it's been his home all his life).
I should feel sympathy that he was caught up in events cleverly tailored to enmesh him instead of wandering off muttering 'What an idiot' to myself, and which sadly colours my thinking of him for my entire playthroughs. If his origin story was better and his whipped puppy impersonation was removed I would like him a great deal more.
Then tie him in more to the main story in the city somehow, certainly to Florrick. But give him something of his own to do, doesn't have to be a major plot thing, just something more personal to him and that if he isn't in your party you won't get and I'd suggest it for Act 2 rather than Act 3. Act 3 already has too much going on all at once and if he is a character you like and have along he does have things he interacts with in Act 3 already, but he has nothing at all in Act 2, total blank canvas there.
