Why I'm not too keen on the whole 'entity cult' thing, personally

It is confirmed that he is the Alchemist, as one of his add-ons confirms that he was the one that experimented on the Killers, with the add-on being the tag that the blight killers had on their back:

A glass-like laboratory tag, moulded from The Fog itself. Either a gift from The Entity or a cruel jest to tag The Blight as he once did to others.

Also, the field of flowers was a vision showed to him when he entered the Realm for the first time. But he spent time as a survivor before finding the notes of Vigo and trying to create the serum, then the Entity sent him to the Void and there he made a deal with it, becoming the Blight.

Not sure about the corrupted versions of survivors (as the blight happened in a very specific way and is clear that he had some potential even as a survivor), but killers turning survivors could be an interesting thing, although it would be very rare, not just because of the nature of the people and beings that the Entity choose as killers, but because it is more probably that the Entity just tortures into submission and if that doesn’t work, throw them into the Void.

However, the devs mentioned that in the first trials, the first killer was a “failure”, so there is a chance of it showing up in some way. You could also do something like a survivor that was chosen as a killer, but resisted the corruption at the last moment and even if they were still trapped and have the temptation to submit,they still have their humanity.

The only correct answer. Ash is the chosen one.

Personally, I love how intricate both Talbot and Felix are to the lore of DBD. Yes, I like the survivors who are just random Joe Schmoes and Jane Does with no predilection to the supernatural, but in a horror game with its main baddie being a cosmic entity of Lovecraftian design, you need to have a select few characters who are familiar with the Entity on some sort of level. Felix and the Observer both knew of the Entity and the Observer even said that it far surpassed what his home planet, Terra Primus, knew of it. Talbot being the first survivor to turn killer that we have playable ingame is also huge because it genuinely shows how dangerous Talbot is. Not only was he capable of kidnapping killers and drugging them with super serum, he escaped the void only to become an addict of the serum himself.

If you want Talbot's full story the timeline is as follows:

*The Blight's Ingame Lore - It ends with the opium den, which is very important

*The 2018 Hallowed Blight Journals - It opens with the same opium den and ends with Talbot injecting himself and getting voided by the Entity

*The Hunger from the first Tome - It opens with Talbot wandering the void, barely alive, finding a flower and then it ends with him promising his devotion to the Entity for more serum

*Talbot is no longer the Alchemist and is now the Blight

What you're assuming though is that both Felix and Talbot are done in terms of potential character arc and storyline, or at least that the outcome that the two will end up in is predictable, trite, and uninteresting. I personally can't change your mind about these things but like I said beforehand, I'd rather have a broad mix of characters instead of the same backstory each time slightly altered. A man who lost his family to the Entity and hates himself for it and a brilliant but callous alchemist gone insane with the Entity's power are both incredibly unique and interesting concepts.

Sure, Felix and Talbot can come off as "important" now and it may seem out of the blue to you but Felix has a long ways to go before he's anywhere near more remarkable than our current cast of survivors and Talbot has had two years of backstory before his release next week to justify his significance. He's already been through his personal hell.