Since there is not an overall difficulty slider to 5e as there will be in BG3, how would you suggest making encounters easier and harder?
*Points to the difficulty settings in BG 1 & BG 2*
Use those. I mean this is supposed to be the third game in a trilogy, don't need to reinvent that wheel.
I don't think that will work in 5e. There isn't a way to out scale hits with AC like there was in 2e. BG1&2's main difference was more damage when hit but in 5e It's not even a matter of a completely min maxed character; you just aren't given the tools to prevent the hits, or guarantee them. You would just die from the increased damage on ever hit. I don't even think spell failure is in 5e, casting or copying scrolls, and that was another annoying trait of the difficulty slider.
The best way to scale encounter difficulty in 5th ed is to add more enemies to the fight. Add some more peons... Add a lieutenant to the boss fight (+more peons)... That said the issue there is it slows the encounter down. On table top the DM generally breaks peon minions into groups and has each group act all at once for speed.