- Don't overthink it. D&D mechanics in 5e have been simplified and they work well at the table or in a video game. Instead of trying to rebalance the whole thing by removing attack rolls/chance to miss why not try to implement functionality that has previously never been done in video games such as grappling mechanics or flying mechanics (been done, but not in a D&D / Party based game).
- Contrary to some others, I feel it would be better if this was a slower paced, low level adventure that then goes on to do higher level stuff in a 2nd game. What better to continue the Baldur's Gate franchise than making the deep, rich story of a PC and his party members that takes us through over 100 hours of game time over ~2 games, each with an expansion or 2? In other words, I'd be perfectly fine with BG3 only taking the PC to level ~7 or 8 and having an expansion to 10, BG4 (hopefully) then would be able to have a higher level adventure that takes the PC to ~18 with a 'final chapter' ala Throne of Bhaal to take the PC to 20.
- Along this vein, please avoid giving out overpowered/end game equipment and then taking it away to start the next game. D&D 5e has brought magical equipment back in to the realm of being rare and, while powerful nowhere near the ridiculousness of 3.x / 4e magic items. It's ok for the party to end the first game with mostly +1 / low level items and maybe just a couple of +2 / mid-level items... In fact 5e has several magical items that don't give enhancement at all, but other useful benefits.
- Please don't give us a D&D story with non-D&D mechanics. It has been SOOOOO LOOOONG since a good D&D came out. It would be nice to actually have it be a true D&D game. Use the 5e mechanics (with feats and racial bonuses/negatives, etc.) as fully as possible and stick as close to the rules system as possible. If you make changes to one part it will impact other parts and make the game end up looking nothing like D&D.
D&D 5e has in my opinion brought the system back closer to what it was in AD&D (1e / 2e) and I am very glad BG3 is being made when the ruleset has seemingly come 'full circle'. I was ok with 3.x, but always felt the emphasis was far too centered around 'builds' while 4e was something I would barely classify as relating to any previous iteration of D&D. 5e is the first time since 2e where I feel they got it right and improved upon the mixture of supporting roleplay and mechanics. I just really hope for a game that embraces this.
While I would prefer grid/turn-based combat similar (but hopefully less buggy than!) Temple of Elemental Evil, I still think it works fine with RTwP as in the Infinity Engine games. Whatever is decided, the combat should feel like D&D, otherwise having a story set in Forgotten Realms would feel like a marketing ploy where they took a unique system that could be in any custom setting and plopped it in to FR in order to capture the D&D market without actually giving the fans of that system a D&D game.