Garthanos
Arcadian Knight
Let's see. My last ongoing 5E campaign, I heavily doctored the system. I basically re-wrote every monster I used. I regularly employed minions. I added auras and forced movement.
I took the average HP, AC, and damage potential of the PCs and used that to establish the baseline stats of the monsters depending on how many rounds I wanted the enemy to survive (usually a maximum of 5 rounds). I upped the damage to make combats threatening and added combat manuevers to make battles seem more dynamic (rather than "walk up, swing a sword, and chip away at a bag of HP until someone dies" affairs). As a result, the group consensus was the best balanced and exciting edition of D&D they had played.
If I were to DM 5E again on a regular basis, I would use this same formula.
Sounds like a bit of work. And one a lot of DMs I have heard about feeling they needed to do something similar.