Standing in front of them?
As I said, I am aware that is not covering everything, but it does cover a great big chunk of "what happens/how things happen in melee" and then flavor/fluff to taste.
We could sit here all day, probably til the end of days, and discuss, "Well, what about
this circumstance?" and "Nuh uh, what about
that scenario?" But going individual round by separate corner case is not going to get anyone anywhere...and we'd NEVER see a 5e get finished.
I suppose its a playstyle thing, but if there are combatants in front of the mage, I never let the enemies just waltz around them to take a swing at the wizard. If they have missile weapons and want to try to shoot passed the melee folks, that fine. But they can't just race by the fighter for the wizard instead...there's 3 dimensions of armored fury to get by first.
Part of the reason a "marching order" was always so important to have/maintain, I suppose...a party's standard operating procedure, as it were.
There's situation-to-situation circumstances, of course. If it's a large or outdoor area and the party just wandered into an ambush from both of their flanks, then, yeah, obviously the attackers will be able to go straight for whoever they want. But in a "face-to-face" dungeon kind of encounter, there's probably not enough room for them to "just go around/ignore" whoever is in front.
There can not, nor should attempt to, be rules for every possible permutation/scenario that might/could possibly occur in game. That way leads to confusion, eventual contradiction and almost certain madness.
The piling/layering of optional modules may add complexity at a rate that any player/group desires...and if your threshold for complexity (and/or madness
) is higher than the group down the street, then have at it and enjoy.
The rules don't have to, again, nor should say "everyone must apply this level complexity to their game." They need to say, "Here is the bare bones basic simplest framework of the game. Any game of 5e D&D uses this starting framework" and go/add on from there.
OA/AoO are not that "bare bones/simplest possible" mode of playing...with or without the use of grids or miniatures. Hence, optional module. Tack it on when you like...tack it on all of the time...don't tack it on at all. Up to you/your DM/table/group.
--SD