DMG Marking

[Cheesegear]

What about instead of the advantage on the OA it instead let you make OAs against enemies that stand from prone

...Are you talking about creating a homebrew Feat, with an opportunity cost?
Or are you talking about adding a global rule to the game - including for hostiles - that has no cost?

What if it made lots of the best spells even more better than they already are?
What if it made Shield Master even more better than it already is?
...No. Don't do that.

make ranged attacks within your reach?

Instead of giving them Disadvantage...Or as well as?

...I can only think from a DM perspective how badly this would hurt players. A dungeon-dwelling hostile with a reach of 10 ft., would ruin a lot of casters. Would that be the intent?

Are you trying to figure out how you can attack hostiles, on their turn, so that they don't get to do anything, ever?
"Any time a hostile does anything, I want to attack them."

If so...There's a reason Sentinel is so good, and there's a reason it's a Feat, and not something you just get for free. And even then, Sentinel still has a few limitations.
Mage Slayer sits in a similar position, but with even more restrictions...And it's still a Feat - something you have to pay for.

"How do I get more Opportunity Attacks?" ...You pay for them using Feats and acquiring Class abilities.