An AMPC who doesn't have a story is a bad AMPC, they can create a lot of plot and have in the past. Yes they don't have all the tools that a DM does but that doesn't mean they cannot create or have fulfilling and engaging stories.
I kind of agree with the sentiment that the described idea, while seemingly gives a lot of freedom for the player, would also run the risk of making flat, one dimensional antagonists. I'd much rather have a well thought out ampc with ideas, goals, and character that is cautious about when they strike, than a half baked concept that wasn't entirely thought out because they didn't need to be in order to exist.
MPC ( or more simply, Monstrous Player) should be a role that you apply for, and instead of the six month timer for one character- you are allotted six months on a team to portray as many monsters as you would like - with vetted approval, having proven your ability to understand, comprehend, and portray them faithfully to the setting.
How would the vetting process work in this instance, especially for people who have never played monster characters before? As it is now, the application is a concrete, thought out character concept, that has to make sense, be supported mechanically and icly, and in some cases has to assess the player's past behavior regarding ampcs. Would this be a general knowledge based exam? A trial period strictly overseen by dms?
Also, and perhaps I am misunderstanding this idea, but wouldn't every monster that is created still in some way need approval by a dm? For one, they still need to set the race of the monster correctly with the correct attributes and all that. And is a singular dm good to give approval with each new concept? As I understand it now, the ampc process is more or less a group decision. I'd be very surprised if that changed into a singular dm looking it over and giving the thumbs up without consulting anyone else. Unless this is to suggest that these players would just be able to do that on their own- of which I feel like treads into dm territory and I don't think dms would take kindly to that, as much as it would probably lighten their workload.
Your AMPC vampire getting railed by the level 20 paladin wouldn't be as much of a bummer if you had the agency to continue in your role. I think this would inspire people to play more of the less sought templates. As the monster, you are going to die, its just a matter of when - and the how is not up to you.
Fair, I get the reasoning behind this. However, if you are submitting for an ampc application, you go into the role with this understanding that it could happen. And while I fully believe that it is the responsibilty of higher level players to respect ampcs and what they're setting out to do and sometimes they don't fulfill that responsibilty, I don't think it happens often enough where ampcs need to have access to multiple monsters just to feel less concerned about losing their monster that they understood was meant to be slain in the first place.
I am not fully against the idea, I understand the thought behind it and I don't totally disagree with it. I would like to see a little more elaboration on how it would all work.