Since it seems most of the other Resident Evil mods have died off either in early alpha stages or (even worse) in the concept phase, I'd like to suggest a new one. This one will be much like the SCP Lockdown mod in some ways, with ways to find the mutations in the wild. Note that these, while quite detailed, are only suggestions.
Umbrella facility: These will spawn rarely across the map, about as rare as jungle temples. They would come loaded with RE-style zombies infected with the T-Virus, as well as a boss creature hidden well within the facility's depths. This boss creature/monster would be a random boss from any of the games, aside from RE7's Baker family and a few RE4 Boss (Saddler, Mendes, Salazar), which are special cases.
Plagas village: On first glance, this may seem like an ordinary NPC village. However, if you happen to strike a villager, get ready for them to retaliate in force. Occasionally, Osmund Saddler may spawn in one, and be a truly terrifying threat to face. He is one of the only bosses that will not spawn in an Umbrella facility, the others being Eveline and the Baker family, with Lucas Baker being a special case, and of course Bitores Mendes and Ramon Salazar.
Ashford mansion: This building rarely spawns, with a weak miniboss of Alfred Ashford spawning within, along with his sister Alexia. Occasionally, a mini room will spawn deep underground, containing the Nosferatu, the mutated form of their father, Alexander Ashford.
Baker family home: Here, Jack and Marguerite Baker will spawn, as well as Evelyn, the main B.O.W. antagonist of RE7.
William Birkin: This is the first of several bosses you can find in an Umbrella facility. He has five different stages he mutates through, with each one being more powerful. As soon as you defeat him, you receive a trophy commemorating your victory, as well as a sample of the G-Virus. You can choose to either destroy the sample or inject it into yourself and gain new powers.
First stage - Health: 100 Damage: 8/10/12
Second Stage - Health: 150 Damage: 14/16/18
Third Stage - Health: 250 Damage: 20/24/26
Fourth Stage - Health: 250 Damage: 22/26/30
Final Stage - Health: 350 Damage: 26/28/32
Alexia Ashford: The daughter of Alexander Ashford, cloned through the tissue of Veronica Ashford, she has infected herself with the T-Veronica virus and remained in stasis for 15 years to allow it to successfully adapt to her body. She'll go through her three main stages, and at her final stage, she'll gain extreme resistance to most weapons, while the Linear Launcher will do severe damage and can kill her in just a few shots. Each boss has a rare chance of dropping a part to the weapon along with their normal drops.
First Stage - Health: 150 Damage (6/8/10)
Second Stage - Health 200 Damage (10/12/14)
Third/Final Stage - Health 300 Damage (16/18/22)
Jack Baker: He may seem like an ordinary man from the outside, but this villain is no ordinary human. Following his infection by Eveline, he has gained super strength, durability, and insane regeneration, and has two main transformations. You'll have to deal massive damage to be able to put down this monster once and for all.
Marguerite Baker: She may have only one transformation known, but she can hold her own. Thankfully, she has an inherent weakness to fire, and sufficient damage will cause her to calcify and crumble. Other weapons will have little effect, and her weakness means a flint and steel will become a lifesaver, as well as incendiary grenades.
Eveline Baker: She only goes through one major transformation as well, but my god she is a tank. Most weapons will do little to her, but a weapon imbued with the E-Necrotoxin can do major damage to her, causing the same kind of death as Marguerite.
Health: 250 Damage: 10/13/14
Lucas: Lucas Baker spawns in a rare special room in abandoned mineshafts, where he'll put up one hell of a fight. At first he'll do major melee damage, then at 50% health he'll begin spewing spores to poison you and attempt to weaken you, then at 25% he gets a major strength buff that increases his damage by 50%. As usual, powerful weapons are rccommended.
Nemesis: A Tyrant made to kill the remaining S.T.A.R.S. members, he is no different from any other Tyrant aside from the Nemesis parasite implanted into him to make it possible for Umbrella to control him. He is armed with a rocket launcher, which does massive damage.
Health: 250 Damage: 24/28/36
Mr. X: A T-103 model Tyrant, he was sent to destroy the remaining RPD forces to cover up Umbrella's tracks during the Raccoon City incident.