
Thanks to extensive practice with the whip, you gain the following benefits:
* Your damage die with a whip changes from a d4 to a d6 at a range of 10 feet, and from a d4 to a d8 at a range of 5 feet.
* Whenever you hit with your whip at a range of 5 feet, you may use your bonus action to force your target to succeed on a Wisdom saving throw, with a DC of 8 + your proficiency bonus + your Charisma modifier, or it must immediately move 5 feet directly away from you, if possible.
* Whenever you hit with your whip at a range of 10 feet or higher, you may use your bonus action to force your target to succeed on a Strength (Athletics) or Dexterity (Acrobatics) check against your attack roll or become leashed by you. While leashed the creature can still move and act normally other than it can not move more than 10 feet away from you unless it uses its action to escape or is forced outside of the leash range by some other effect. While a creature is leashed by you, you can't attack with your whip but you may use your attack action to either pull the creature 5 feet closer to you, deal 1d4 bludgeoning damage or give it disadvantage on its next Strength (Athletics) or Dexterity (Acrobatics) check. When a creature uses its action to escape, you may use your reaction to force it to succeed on a Strength (Athletics) or Dexterity (Acrobatics) check, against your original attack roll, or it fails to escape.

I'm mostly just commenting on these like the rest of my nonsense because why not.
Whips are generally terrible weapons, I firmly believe that. However, that doesn't stop people from wanting to use them because they are cool and fantastical and stuff. So when a player of mine walks up to me and says "I really want to use a whip because they super cool but I feel like I'll just be way better off with a rapier" and then they give me sad pout-y face, I feel like I should make a feat to make them a bit more competitive. So I do, and this is it. Goals: competitive bump to damage, interesting "whip" related nonsense that makes them feel cool and effective. Balance is questionable but I don't necessarily feel like its better than the rather comparable adventurer dual wielding rapiers with the Dual Wielder feat.
The die increase is very generous imo, almost too generous, but at least it theoretically gives you interesting choices in whether or not you want get into melee for extra damage while giving you a general damage bump. So then we ask ourselves: what are interesting things that people want to be able to do with whips that might almost kinda make a whip an actually effective, if not a terribly damaging or lethal, weapon.
First, the concept of keeping something back with a whip. Actually, pretty self explanatory and rather reasonable imo, at the cost of your bonus action. Possibly useful for disengaging from a single creature and, at least attempt to, keep some distance.
Second, the idea that you could get a creature caught in you whip and keep it there, drag it along with you, choke it etc... A lot more tricky, with some interesting wording and mechanics to make it reasonable, also costing a bonus action. You hit and attempt to leash it, which is like grappling it but doesn't stop them from just running at you and hitting you and is theoretically easier to escape unless your willing to use your action/reaction economy to keep it there. They contest like normal as if you were grappling them except their check is contesting your attack roll instead of a separate check. The higher your attack the easier it is to leash with it; think leashing someone by the arm or leg vs leashing them by the neck. The only thing this really accomplishes is stopping them from running from you and giving you some unique "attack" options. You can drag the creature at a rate of 5 feet per attack, strangle/crush it at a significantly lower rate of damage or keep a good hold on it so that it is harder for it to escape. Keep in mind, if it uses its action to escape, it automatically does so unless you use your reaction. If your willing to use your attacks and your reaction, however, it is very hard for it to escape. It can still at a moments notice turn and start wailing on you though so caution is warranted. All things considered, it's dangerously close to making an almost under powered weapon over powered but there's still a pretty significant chance of failure followed by getting mauled by whatever it is your trying to manipulate with your whip.