The Flanking optional rule in the DMG is the epitome of WotC going overboard in bringing simplicity and reducing stacking numerical bonuses. I get the desire to eliminate as much of the 'wait, I have a +1 bonus from X and a +3 bonus from Y, so my actual roll is a total of Z" that happens frequently at the table, and math crunchy games like PF(x) and 3rd Ed certainly drive me to distraction when nearly every turn is peppered with 'wait...' instances.

But there are definitely times when advantage is just too strong, and a small bonus is more appropriate. If I had my druthers, I'd make it so that you can have 2 instances of a bonus to a single roll, outside of Ability Mod and Proficiency Bonus (those always count). So, attack might have Magic Weapon, Fighting Style, Flanking, Bless etc. And you can pick any two of them. If you manage to get 5 bonus options, you still pick 2, but you get Advantage on the roll. Doing this, I'd also change Reckless Attack to a static +5 to hit (and enemies get +5 to hit you). It's another bonus Barbarians can pick, and it doesn't become a class tax for crit fishing builds (nor a trap for Rogues).

Regarding OAs, I don't understand the mentality that you either have 3rd Ed style movement and a lot of AoOs, or 5th Ed style movement and 1 way to generate an OA. Like, there seems to be this psychic divide that you can't have movement outside of an action type and detailed OAs. The only thing I can think of that perpetuates this concept is that AoOs stopped movement. But in 5E, that requires a feat. Sure, it might make Sentinel a feat tax for tankier martials, but I don't see why casting within 5' or using a bow within 5' isn't punishable by more than disadvantage (and for spells, only if it has an attack roll). You wiggle your fingers within striking range, and I'm gonna try to slice them off!

There's been a bit of discussion on how easy it is for casters to get heavy armor. Well, granting OAs to casting within 5' instead of allowing them to wade into the middle of a scrum and PBAoE all over the place might curtail such multiclassing/feat expenditure naturally instead of making some weird rulings about it.