If starting from scratch, what would I take from each edition as a basis?
0e/Basic/BECMI (these all kinda blur together for me) - short spell lists, simple randomized char-gen, stand-alone adventure modules, general sense of whimsy and-or gonzo, lethality and danger are a constant fact of life, underlying philosophy of guidelines-not-rules
1e - the PC race selection, most of the pre-UA classes plus Cavalier (specifically including the 1e Ranger!), clerics-v-undead subsystem, individual weapon proficiencies, variable xp progression by class, strongholds etc. at higher level, training rules, risky magic e.g. expanding fireballs or possible death in teleport, saving throws that depend on what is being saved against
2e - open-ended but slow class advancement, death at -10, many spells (but severely curated for each campaign or style), magic item compendia, alignment, slow recovery rates for spells and hit points
3e/3.5e - sorcerer casting system (slots but no pre-memorization) for all casting classes, Bards that can start at 1st, NPCs and PCs on the same chassis, various magic items
4e - geographical and tactical set-piece encounter design, terrain rules, bloodied condition for everyone, points-of-light style setting
5e - advantage-disadvantage in certain situations, bounded accuracy but only to a point
There would be a base 1e-like set of classes intended for all games plus a similar-length list of options that could be added or not to any given campaign or setting.
I would also harvest a selection of monsters from each edition's various MMs - hell, what am I saying? I'd just use them all.
