I would consider putting in Carrion Feeder . The synergy with Marchesa is too good to ignore. Sure it can't block, but then maybe you'll want to replace it with Bloodflow Connoisseur . Either way, it is important to know that both of these creatures protect themselves because of their +1/+1 counters.

Both good ideas... I'm not 100% on Feeder, but I am looking into things that may lower my curve.

I was missing removal. Added a bunch of removal + a bit of control, and now it does very well.

Where's the +1+1 counter love and alternative sources for those +1+1 counters?

Look into Oona's Blackguard, Metallic Mimic, Herald of Secret Streams, and Drana, Liberator of Malakir. Also look into picking up Sneak Attack and Mikaeus, the Unhallowed. Neither is cheap but both are worth their weight in gold here.

Feel free to check out my list for inspiration. He who controls the Spice controls the Spice Girls Feedback, ideas, and upvotes are appreciated.

Ashling the Pilgrim: Doesn't really do anything here. Sure, you can boardwipe, but there are better things for this.

Bogardan Hellkite: Eight mana for the effect? This is overcosted. Consider Inferno Titan or Ryusei, the Falling Star.

Dack's Duplicate is in a lot of Marchesa lists, and it shouldn't be. It's better in almost every way to run things like Captivating Crew or Sower of Temptation.

Drana, Kalastria Bloodchief Kinda just a goodstuff card, with a commander that doesn't want to be running goodstuff.

Duplicant: Allow me to introduce you to Patron of the Vein.

Niv-Mizzet, Dracogenius: Again, why is this here? This ain't a goodstuff deck.

River Kelpie is a trap. I have played this card EXTENSIVELY and I find it to be a "win more" card. It ONLY works when you have your board state set up, whereas Harvester of Souls or Dark Prophecy gives you cards regardless of whether or not the things that died slipped through the cracks.

Shadowborn Demon: It's a goodstuff card that isn't even good. Run Vindictive Lich or something else that actually has synergy with Marchesa.

Finally, why don't you have any sac outlets? Ashnod's Altar, Attrition, ect. I feel like you're not entirely grasping what makes Marchesa powerful. You need to find a way to build around her and abuse all of her abilities. I'm not even recommending my list, just that you look at what stands out about the general and go from there.

Things that are a MUST HAVE in any Marchesa, the Black Rose deck:

Purphoros, God of the Forge: He's a win condition that is nearly impossible to deal with.

Mikaeus, the Unhallowed: Your creatures are invincible and you can sac them twice per TURN.

Sac Outlets: Ashnod's Altar, Yahenni, Undying Partisan, Attrition, Grimgrin, Corpse-Born, Goblin Bombardment.

Blood Artist, Zulaport Cutthroat, Skullclamp...things that do stuff when other stuff dies.

+1/+1 Counter Synergies: Herald of Secret Streams, Drana, Liberator of Malakir are both amazing.

Optional: The Modular Build. Run every single modular creature plus Triskelion, Walking Ballista, Hangarback Walker. Your creatures can NEVER die for good and you ramp up to a very solid win.

What he said.

Haste, +1+1 counters, and abusing etb/death triggers are what make Marchesa tick. Shes a synergistic nightmare for your opponents when built right. Ogre Battledriver, Anger, and Urabrask the Hidden are all good haste enablers, both of us listed some good +1+1 counters cards, and the list goes on and on for etb/death triggers to abuse here.