How about flanking for advantage, something that is an optional rule for 5e (not the "I get advantage if I'm behind you" backstabbing BS from before, but proper 5e flanking option).
You mean that you should get Advantage on Meele attacks, when anyone else from your party is in meele range from that NPC?
That is certainly there, it was dicuised in "Patch 5 news" topic, since Larian removed "auto advantage when attacking from behind" but changed it with this.

I actually might want to try getting around behind them with some of my characters to trap them between the two positions hammer and anvil style. I can technically do that now if I'm so inclined, but because of high ground giving advantage for no real reason, it's more advantageous to just keep the whole party on top of the high ground and use the strength characters to push anyone who comes up back off while flinging arrows at them.
So ... your point is actualy not the fact that you want to "figure out" some clever tactics ... but that you want those tactics to be more effective.
Since as far as i know, there is nothing stoping you using what you described here. O_o
In fairness, I'm just not sure what tactical and clever strategies one can reasonably utilize at low levels aside from seeking out positional advantages.
That was kinda my point.

I maybe hoped a little that someone mentions that he would like to see option to "hide" behind objects (such as Barrels, Big Rocks, Tower Shield, or simply bigger teammember

etc.) ... wich would be negated by High Ground ... that is tactic i would certainly welcome!
But im not quite sure if that is even in Larian power to implement this properly.

Personaly i would like to see that any "tactical" decision we made will have some consequences ... or risks if you want.
Few examples:
You want high ground on the hill? No problem, but every single enemy will imediatly run as far from you as they can ...
You want high ground on some raft, or simmilar construct? Nah ... bad idea, one shoot here, one shoot there and construct is falling appart (note that i mentioned at least two attacks) ...
You want to destroy webs in Spider Queen lair, when she will be on them to get fall damage? Good choice, except spiderlings have incerased speed on them, and can get to you ... you have to decide, set a trap and risk ... or burn them to block spiderlings, but loose a trap.
That is kind of "tactics" i would like to see.

But what's perhaps a bit over the top is just how far the strong characters can shove people. A couple of meters seems reasonable, maybe a bit more if the strength difference is massive and the target is light, but after that the target should fall downwards not further outwards, regardless of verticality. At the moment, it seems like the horizontal speed is preserved all the way to the ground, which makes some shoves truly ridiculous.
Honestly i kinda like one suggestion that was here for Shove ...
Someone suggested that Shove should knock enemy prone on sucess ... and throw them away only for crittical hit, and even then only 1-2m ... that seem interesting, reasonable, and more than usefull for meele characters (at least to me)