D&D 5e Monster Campaign

[Talon Betimere]

Greetings travelers,

So a friend of mine is trying to resurrect an old 3.5 campaign idea of letting PCs play as monsters. As far as I know, he's allowing us to pick a monster we want to play, starting at CR 3 (Lvl 3) but with a CR 5 cap, meaning if we picked a CR 5 monster he'd tweak the Hit Die and Ability Scores to better match a CR 3 to my understanding. Beyond that, we both hit a wall as to how we would level up our Monster PCs. Unlike 3.5, there is no more LA (Level Adjustment), at least none that I can see, and obviously just gaining Hit Die would grow tiring and boring since there would be no more added flavor as with class abilities, just us getting more beefy.

We tried thinking of examples for certain monsters and we thought of one for the CR 3 Spectator that once as a spectator levels up, they use some sort of currency gained from experience to grow another eye-stalk and thus gain a new eye ray until inevitably becoming a true Beholder. This was something we spit-balled in like 10 minutes, but it's an example of what we could POSSIBLY do.

My question now is, what do you guys think? How could we make this campaign work? We both realize there's very little to no material on this, i.e Savage Species, and are okay with the idea that this could require a lot of work and converting. We just need the ideas and suggestions to push us into the right direction.

[lebefrei]

What's wrong with gaining class levels? Start as CR 3, pick a class after X experience. If you start as CR 4, you can't pick a class until X+Level 1 exp, and so on.

Add character levels to your monster, with a cap of 20-CR to keep some semblance of balance. That should work out decently, I'd think.

[ShikomeKidoMi]

I think it's doable but it's going to be very dependent on what monsters your characters pick. Spectator to Beholder is a good idea and you could probably do the same with someone moving up the food chain of incorporeal undead (if you don't find incorporeal too problematic a quality). Devils and demons come with ready made advancement paths, though they might be hard to mesh with the group.

Or, as the person above me said, you can add class levels to weak monsters and work out some kind of CR-level adjustment equivalency.

[Ninja_Prawn]

Depending on how much homebrew you want to do, I might advocate doing full conversions from 'stat block' to 'PC race'. It's laborious for higher-CR monsters, but I've seen it done for succubi, so it ought to be possible.

Of course, that will mean a general stripping-down of what the monster PCs can do... If you wanted to go even further, you could repackage the overpowered abilities into prestige classes that would, in the long term, function kind of like a Level Adjustment. A lot of work but, if done well, it would be the best way to preserve balance.

[Addaran]

Easiest would be for all the players to pick something of the same CR then just had clases level normaly with time. It will be harder to gage the real CR of the team, but they should be equal in power.