I'm planning a campaign with some friends that has some creepy elements to it. Inspired by the new Vita Carnis, as well as other scary creatures like those from Resident Evil, I had an idea: The Carrionoids. However, I'm wondering if this might be a little too much to keep track of, so I wanted to hear some feedback.

The Carrionoids are horrible flesh monsters that get steadily more disturbing the longer they are allowed to grow. Their growth is determined by a system I've called Global Growth Points, or GGP: Basically, certain actions from the Carrionoids and the PCs can make this number grow or shrink. A Carrionoid kills someone? Add some GGP. The party kills some stronger Carrionoids? Subtract a bit of GGP from the total.

The reason that GGP is so important is that it determines the current Phase the Carrionoids are in. The more Phases progress, the stronger the Carrionoids get, and the more interesting attributes they obtain. In the early phases, they might be nothing more than pests, but at a point like Phase Eight... Well, then they're well on their way to world domination.

What do you think of this concept? What suggestions do you have? I'd love to hear what you have to say about these potentially world-ending meat monsters.

I would find the setup you describe a little too limiting. For me, the point tracking would get in the way of dramatically appropriate moments for the Carrionoids to become more or less powerful... But that's just personal preference. The concept is great, although I know I as a player would end up creating a character that would ask the question, "Where do the Carrionoids come from and what do they want?"

That could work. It’s not that different from saying, the villain’s plan will progress as follows unless the PCs intervene. 
I also agree with eapiv about it maybe limiting yourself. You can always do both. Track the points, but if you decide they’re moving to slowly or too quickly adjust as needed. 

I love this idea, but I can imagine it getting a bit frustrating without adequate reward for lowering GGP. Players and their characters have to feel that their quest isn't entirely hopeless; even grimdark mitigates exhaustion with some glimmer of hope. You use Resident Evil as inspiration, so you might be familiar with the video game series XCOM: the aliens will strike three zones of the 10(?) zones in the game, but only one of them can be dealt with at a time. In doing so, panic (GGP) is reduced in that zone but it rises in the other two. Eventually some zones will totally succumb to the carrionoids, which reduces the total amount of difficult choices but also means the failure state draws nearer. Each zone offers different benefits, losing a zone on the world map to carrionoid domination could have devastating effects on the ability to see the war won.

In XCOM, total annihilation of the aliens means understanding them. Being able to gather samples from the stronger monsters means better arms and armaments against them, and if they're an intelligent life-form, a means of communication. In D&D terms that's +1/2/3 bonuses or additional damage die, or maybe characters might be willing to undergo carrionoid corruption to better understand and defeat their foe (see Warhammer 40,000: Dawn of War - Chaos Rising's corruption mechanic), at the cost of boons from their gods or stats.

XCOM is also a game that has a heavy focus on downtime, as you establish satellites to reduce panic, research things, train up soldiers, that sort of thing. It's very West Marches, and it could be a format that works for you: maybe you have a player who would be better suited being back at base, and they bring someone else more specialised for this operation (is an archer going to do well clearing out each shack in a shanty town, for instance? Will the alchemist artificer be better suited to collecting samples?). You'll also be building your base, which could be a funnel if you give out too much of a monetary reward. Put it toward shoring up defences, paying mercenaries to escort traders or makeshift ambulance services, or bringing along hirelings to do some of the heavy lifting.

That's all I've got for the time being. I think you've got a great concept, and I'm sure you can execute it well. I wish you luck, but if you want us to bounce any more ideas off of each other I'd be happy to help!