I'm planning on eventually coming up with some way to convert monsters into PCs. My thought is that you start with a monster and it counts as a certain level. Then you can add class levels or maybe more monster levels. Have to figure out what changes are made when it converts to PC.
I'm not primarily talking about lower CR things like playing an ogre. I'm talking about a full fledged adult dragon, or medusa, or mind flayer that starts with all the hit points and features from the MM and then goes from there as part of a monster campaign. There is no weakening monsters--you just can't play them until the campaign level is at least as high as the monster.
The biggest problem I've had with it is how to accomplish both of my goals:
1) Meaningful character customization and advancement
2) Fits the general 5e style of how monsters work
In 3e it wasn't terribly rare to run into NPC monsters with class levels. Having a medusa that was also a 7th level sorcerer, or a mind flayer with 13 psion levels are things that happened in adventures and settings. In 5e...we don't see that.
5e occasionally gives NPC monsters some extra features to their statblock (this chromatic dragon gets the change shape ability like a metallic, this storm giant casts wizard spells much like an X level wizard), but just adding on hit dice and class abilities doesn't happen.
If I were to let it work like 3e, I'd feel like I were changing the way the world works and should start making NPC monsters like in 3e, or I'm saying PC monsters are really super-duper, on a whole different plane, exceptional. That PC isn't just an adult bronze dragon with a little higher Wisdom score, they also have 7 layersbif ranger stacked on, while no monsters are like that.
So, basically, the MM dragons become just "basic" dragons, while advanced versions apparently exist, but since I don't want to mess with it the only one ever seen is the PC.
And besides the weird rarity issue, all dragons really should be awesome. You shouldn't have any boring standard great worms compared to the extra special ones. I don't want a PC dragon to be substantially better than an NPC dragon. But how do I provide meaningful customization and advancement without doing so. (I'm using dragons because they are an excellent example.)
The idea I'm currently toying with is to kind of figure out what the stat block would look like for an advanced NPC monster of a particular type. As much as possible, this means finding actual ones that already exist in the game. See what sorts of features are and are not given. See what the most powerful one looks like. Then find ways for PCs to advance within that framework.
So maybe a PC giant or dragon, can only gain levels in certain classes (or a monster-specific version). They don't get any hit dice unless the published advanced monsters did, but they probably get some features with each level. These could include levels of spellcasting, special abilities (maybe treated as feats), and attribute increases if the advanced monsters had better stats.
For initial customization, rather than point buy with ability score adjustments, something more like you take the stats from the MM but can move some up and others down using a system similar to point buy, but with limited range. You probably can't raise a giant's strength by much (those strength scores are strongly associated with specify giant species) if any, but you could drop their one or two of their stats by a few points and raise there wisdom a bit to let them be better with the cleric spellcasting you are giving them.
You can probably pick some extra languages or tools, and swap some spells. Maybe pick up weapon or armor proficiencies. The minor stuff that could be assumed the monsters in the MM might actually have but it didn't make it to the statblock because it wasn't relevant for encounters (tools, background features, etc) could be added on, while other stuff would likely need to be swapped for something the monster already gets, and should probably be a bit unfavorable except when considered in proper context. Losing 4 points of physical stats for 2 or 3 points of Wisdom for example. Good for your clericy giant, but there is a reason typical giants have their stats where they are. If course, when we convert the monster to an equivalent class level, that's almost always going to be a higher level than its CR, so the proficiency bonus will be higher for the PC class conversion version.
For something more like an ogre, I think it could be a mix between that and just adding class levels, since even in 5e having an ogre that's also a 10th level barbarian wouldn't feel too terribly out of place, even if they haven't really published them.
Probably still have a more restricted way of determining ability scores, and have your initial levels overlap/replace rather than just adding to the monster. I'm not sure what to do with max level characters. For some monsters, they absolutely should be able to have the equivalent of 20th level class features and extra monster power and hit points on top. For others, like ogres, it seems like maybe their monster levels should be at least partially included in a max of 20 limits. Should an ogre 20 th level barbarian be more powerful than a half-orc 20th level barbarian ( if played alongside each other that would mean the half-orc has advanced through epic boons to be the same XP total)? Or should they be limited to a 17th level barbarian with ogre counting as 3 of their allowed levels? The latter feels a little better to me.
Anyway, it's a tricky mess, but I think can still be worth coming up with a system for, because it can enable fun campaigns.