Regarding BG1, BG2, I must say that I had a love & hate relationship with the reputation mechanic. Personally, BG1, BG2 really handles well the reaction of the game when you do evil deeds.
When you do bad stuff:
-reputation goes down
-price of goods goes up
-encounter with guards increase
-evil chars get happy, good char get angry
-You can recover reputation by performing good deeds, or paying money to temple
From what I remember, we really felt the consequence of bad deeds and I felt great about it.
However, I always felt the best thing to do was to create a loyal good char to get as much reputation possible to get lower price possible and avoid encounter with guards.
Most of the time, one of the only bad thing I did was killing Drizzt for his nice stuff and then paid a bunch of money to the temple to get 20 reputation once again.
For the most part I would keep reputation mechanic the same but yet I have a huge concern with previous rep mech.
Was there any advantages of playing an evil char in previous BG games apart of having few NPC on your side?
What I would like to see in BG3 is more ways to benefit evil chars (Maybe: can access black market stuff, join weird secret guilds, maybe skills and spell that can only be used by evil chars)
Summary:
I would like a more equitable distribution of consequences/advantages for each alignment. I would like that you have ways more benefit to playing an evil char and more consequence of playing a good char (for example, can't sell stolen stuff if you are good and things like that).
Your view on this?