Bonus XP for killing monsters and getting dressed with their skin so you can frighten their young

I Love the money=XP system usually used in OSR games I also love systems like carousing, as they give the right flavour I tend to go for in my games: The characters aren't heroes, they are mostly the brigands we get to see the whole story unfold. And as such I think these stories should add mechanically to the character's growth as well as being a system that facilitates more play. But I hate just giving out XP after the players just kill something.

You mean your players don't keep the ears of the elves they kill?

Source: LotFP

"So, you said you killed the beast of Braun Manor? Can ya prove it?"

I like to assume that when my players are carousing they are probabbly sharing the tales of their past adventures with the local scum, and that's part of the abstraction that the money they are spending in their revelry is adding to their experience. They are showing off and paintign themselves as better than they really are, that's why next time they venture out they get to hire better suited people for the job.

But boasting and bragging can only get you so far, maybe the local scum will want you to prove that you really felled the dragon that was pestering the farmers down south, and those scars aren't cutting it.

So what will it be mr. adventurer?

You belive that the orcs are dead now?

Source: RDR2

The system itself or "I am a murderhobo, you can tell me by the way I walk"

Killing a monster and taking a trophy from the kill will net you HDx25XP, maybe more if your DM feels like it. Thing is, this XP is only with you if you are carring the trophy in a way that it's showing it off. And if someone else decides that fashion statement would be better suited to them, well the regular brigands just gained a form of Level Drain. 

But Wait there's more!

Everybody back in town knew the random schumck that was walking around town wearing the orc cheiftan's face as mask, and word will certainly get around, but now, now some other random schumck is walking around with the orc's head and people around other parts don't know your face, they just know your name and your quirky sense of style and that's great for the assassin that just stole your "identity" as he can frame you for his next crime.  And now your players have all the more insentive to go after whoever stole their trophy.

Maybe instead of just bailing out the badly payed and mistreated hirelings steal the giant's head that the party was going to gift the baron to try and get some favours
Also, you added a element of inventory management to XP.

I'd also give them a bonus/penalty on reaction rolls based on what treasure they lug around. Other Orcs could now be more hesitant to attack since you killed that famous chieftain.
Maybe it's now easier to hire hireligns in the region, since you are the one responsible fro killing the beast that was attackign the farmes/caravans.

Scaling things up or "the manticore pelt rug really tied the room together"

Source: Oglaf (NSFW)

Sooner or later your Players will find someplace to call home, It could be a keep, a manor, a ship or even tehir very own circus caraven. Maybe they'll need a place to put down all those goblin heads as showing off the same five old rotten heads everywhere they go is getting pretty old, or caring around a hydra's head is a little impratical. They should have a place to keep their trophies relatively safe. They should have a trophy room.

Source: Bizarro

Basically give them some place to store their trophies, make them carry around that giant piece of beast while in a weakened state and after they arrive in town let them celebrate their victory and add the XP for the kill after it is paraded around and stored back home. Going to the dungeon is a logistics challenge, clearing it and coming back are also part of this challange and now, you can add another layer to it as well as giving your players some more presence in the world.