I am unapologetic about thinking about game balance, session pacing, directing players to fun, and other deviances from old-school orthodoxy! (I agree with you in general about later editions, although I think the problem there is not so much balance in general as balance along the single axis of combat effectiveness.)
But to get that balance, we should do a more complete job of thinking about what abilities you want to have as a cleric, and which as a doppleganger, since we're not working within the Moldvay race-as-class package Eric detailed so nicely.
In order of definedness:
- clerics can use heavy armor and shields, use medium non-edged weapons, turn undead, cast cleric spells, use cleric items, are favored by the gods
- characters start with a special power
- characters get some abilities from their race, which are the least obviously balanced part of the package, ranging from "humans get nothing in the rules but the current age of creation is meant for them" to "elves get lots of things in the rules like infravision and secret door detection, some pay more XP to get AD&D powers like immunity to charm/sleep/paralysis, but they're responsible for the dreamplague and are especially tasty to bulettes"
As it stood in a very off-the-cuff way, I was thinking of you as having:
- everything a cleric has
- a special power that lets you disguise yourself and do minor Plastic Man stuff like swallowing items you want to hide, reshaping your wrists to escape from manacles, etc.
- racial abilities that give you natural armor as chain without its encumbrance and general dopplegangeriness (e.g. recognizing your own kind, knowing about doppleganger culture, etc), without so much elf-jazz that'd I'd feel obliged to go out of my way to make things difficult for your entire species
And I also normally envision a doppleganger as a kind of amoeba that walks and can change its integument, so that its changes are neither magical (otherwise you can pick 'em out with detect magic) or more than skin deep (so that the elf-looking retinas don't start seeing into the IR spectrum, ogre-looking body doesn't really grow more muscle mass and is in fact maybe a little less proportionally than when you're in halfling form).
I'm happy to negotiate about and around these assumptions; lay out what you want different and we'll dicker!
