A good start would be to leave spells and abilities as they're designed in the 5E rules instead of adding surfaces and interactions that turn every fight into a series of explosions and environmental effects as early as level one. I've never had a level one encounter in D&D where I was more worried about a barrel than I was about a goblin. That's just not what D&D feels like.

When the Infinity Engine games changed a spell or mechanic, it was typically because of an engine limitation, not because it would serve some subjective purpose or to leverage old design/engine work. If you want to make an original RPG, please do so, and please call it something else.

BG3 is a ton of fun as a game, but when you start adding environmental effects to the already hotly debated at-will spells casters get and give disadvantage/advantage on all attacks to .1 meters of vertical disparity, it stops being D&D. And for those of you out there who don't really get what people are on about, you should understand that Baldur's Gate isn't just another quality cRPG like Mass Effect or Dragon Age. It was beloved in large part because it was so tightly analogous to playing the tabletop game at home without friends. Those of us who played this series growing up and have fond memories of it typically do because it was so much alike to the paper & pen system, something to fool around with between campaigns. To throw that out and still call it Baldur's Gate is really silly.