kenmarable
Hero
For PC use, personally, I think it should be 100% an entirely different process applying a template. With 3.x, initially I loved the idea of monsters, NPCs, and PCs all using pretty much the same leveling mechanics where "Fey" or "Dragon" were basically classes. Along those lines, Savage Species is one of my all time favorite 3.x books. (I like to read new Monster Manuals as sources for PCs.)I think you're both focusing only on NPC use of those creatures. I am trying to see both sides, NPC and PC, even tho the latter may be useful to less gaming groups. For NPC use, see my post #7.
However, as much as I love the idea and the nice symmetry of it, in practice, it falls apart pretty easily. With monsters/NPCs and PCs playing entirely different roles in the game with very different needs, often something worked well for monsters but poorly for PCs or vice versa, or even worse was balanced for both by being kinda sucky at both.
Now, I definitely want something that allows us to templatize/monsterify/whatever PCs. However, I would prefer the rules on how to make a PC into a vampire (or whatever) weren't just take the monster stats and apply to your PC. I'd rather see very specific information including fluff on what the process is like, different stages they will go through, how to balance the powers against the rest of the PCs (assuming they are not all vampires, of course), specific things to look out for (e.g. climbing becomes irrelevant), possible impacts and interactions with the RP parts of 5e like traits and ideals, etc.
In other words, I want information (not just rules) dedicated to how PCs can gain templates and not have that just be the monster reverse-engineered. With that, I'd rather it was broken up to work for PCs even if that is very different from how it works for monsters. AKA Savage Species where the powers gained may or may not have anything to do with the original monster write up. I'm fine with a PC human vampire being very different from an NPC human vampire. I'm ok with breaking that level of verisimilitude if it means the rules for monsters are ideal for monsters and the rules for PCs are ideal for PCs. At least that's what I'm hoping for.
(Which, I admit, kinda sucks. With 3.x, it was so easy for them to just add in "LA: +4" or whatever and make so many monsters playable as PCs which gave someone like me so many more options. But having been down that road, I'd rather it was more thought out than that even if means getting more guidance on how to do it myself rather than them making it clear with a one-size-fits-all rule.)