I didn't forget about this @Charmander! Just needed to give it some proper thought and the time to respond.
That being said, if you're looking to create agency and purpose for players without a PR, I'd recommend giving Vanillagers a shared power, specifically a shared Night Action. I've utilized this in my Nintendo Switch Mishmash and UD has utilized this concept countless times. In short, you allow Vanillagers to work with a village PR to Vigilante Kill, or Roleblock, or Angel, etc., a player of their collective choice. So long as the Vanillager PM is public knowledge for the wolves to be safely able to claim, it's a decent alternative. I've also as recent as the Mob Psycho 100 Mishmash utilized roles that create synergy, such as the Wolf Hunter, which gains action based upon the number of Vanillagers that die, or Mob's role, which gained a Day Vigilante Kill after a certain number of villagers died. These are more indirect methods of involvement but add purpose all the same.
The short of it, unfortunately, is that the best way to provide agency for Vanillagers without giving them actions... is ITAs. Or pseudo-Night Actions. This is definitely an area of game design that I believe needs more innovation. Because the contrary, Role Madness, where everyone and their dog has a role, is lazy design, in my opinion, and can create several game-breaking issues.
This may be a cop-out answer, but I don't think it's possible. My first piece of advice would be to understand your audience. In my experience -- and I mean no offense to anyone, but -- I've found MU is generally heavy/complex mechanic-adverse. Most players (not all) would rather receive a simple role with simple expectations and shoot players then die and laugh about it in DVC. Which is totally fine. Compared to another forum, like POG in its day, where complex mechanics were mostly expected. It's cultural.
If complex is the way you'd like to go (and I highly encourage it: it's more fun and makes players think!), my next piece of advice would be to have several other experienced hosts take a look at what you've come up with, AND find a way to explain it in its simplest terms. The way a PM is written matters greatly to the success of and understanding of a new mechanic.
I think that's a fair assessment regarding AI role complexity. I design my wolves to have complex roles that work especially well with others, but are never useless on their own. Villagers' roles can be individually complex in ways the wolves cannot because the loss of a single villager is not nearly as detrimental to their wincon. I'd also add, that wolfing is difficult, so I try to make the collective complexity of my wolf roles lesser than that of my villager. Forcing the wolves to do a song and dance, then run a lap, just to use a simple mechanic adds to the stresses of having to lie for 12 hours a day.
I think it's preference, really. And most definitely a combination of all of the above. No method is superior than any other, unless that method is using a bit of each. Because if, for example, a game utilizes only x-shot actions, this can open the doors to a mass-claim and solving through the gaps. So unaware or aware backups help. But having only backups can equally screw over an alignment if a single PR dies and the backup has already died or is unequipped to handle the added responsibilities.
Also, frankly, as a game designer it is difficult to create 25+ fresh roles for each new large-game. So utilizing a little bit of everything to create an asymmetrical design with plenty of support to streamline actions is the best way to go about it.