Delta World
While I love old-school Dungeons and Dragons, that brand of ten-foot-pole poking, door listening, dithering over every door to keep your 3 HP fighter alive is not always what I want, especially out of a one shot. Is there a way to play this game with hardier characters, eschewing some of the sacred cows like levels and character classes, without sliding into simulationism or heftless microgames?
Enter Gamma World, a less loved product of TSR about exploring far future, post apocalyptic earth. The first edition of the game (and Metamorphosis Alpha before it) is essentially D&D at the bones, and honestly reads more like an OD&D supplement than a complete game. And while GW concerns itself heavily with technological relics and mutant powers, its take on D&D mechanics is novel in its own way.
The other night I had an opportunity to run a one-shot, and wanted something more hack-and-slash. Still lightweight and shaped like OD&D, but without slow trap checking and piles of pregens for replacement PCs. Thus, I threw together DELTA WORLD. The goals of this little OD&D/Gamma World hybrid are simple: pulpy player characters with more HP to burn as a resource, using the pre-existing rules of D&D and GW where I can to avoid writing anything new, and compatibility with any old D&D adventure.
Character Generation
Take 3d6 down the line (or 4d6 drop the lowest, I won't tell) as your normal D&D stats. Strength gives a +1 to melee damage for each point above 15, and a -1 for each point below six. Ditto for Dexterity, but the bonus applies to all to-hit rolls. Constitution is the number of d6s you roll for your hit points. Wisdom will primarily function as a roll-under for saves vs. spells. Intelligence works like it does in Greyhawk. Use that table to learn new spells and figure out how many spells of each level you can learn. For Charisma, determine maximum number of hirelings, loyalty, reaction adjustment, etc. from the table in Gamma World or your favorite version of D&D.
Referees are encouraged to rely up on PC attributes when making rulings, just as with OD&D.
For combat, use your favorite algorithm (ascending AC, target20, whatever). This might be a good chance to use a Weapon vs. Armor table if you can stomach them, as the players may be hunting for more bonuses without levels to boost their to-hits.
Saving throws are a primarily roll-under-stat affair.
Thief skills are very much in play, and go up through use. Succeeding and failing a specific skill will up it to the success chance of the next level of Thief. The skills increase independently.
Magic is found and learned using the table in Greyhawk, available to all. Maybe start with some spells if they have high intelligence. Casting is still restricted by armor.
I haven't used this for campaign play, but luckily Gamma World does come with some advancement. XP can be gained for normal old-school stuff as you choose, but all leveling up does in Gamma World is give a random, small stat increase.
I want to use D&D adventures, so monster stats are going to be unchanged. Yes, this means players will have way more health than the average monster, but that's alright. They may still struggle to hit and be overwhelmed. As saving throws are roll-under-stat in Delta World, a quick rule of thumb is to have monsters save on a roll under their HD + 5 on a d20 (ripped from Whitehack).
How Does it Feel in Play?
So far, characters are hardier, but not as immortal as the huge HP might lead you to believe. Remember, they still have the equipment and stats of a normal level 1 adventurer. My recent one-shot still had a character cut down between a fight with several 1 HD monsters and stepping into a trap. If we were playing OD&D he'd have gone through a character each there, and someone else would have probably died too.
The big upswing is that it alleviates my biggest gripe with old D&D, which is that the (much maligned in 5e spaces) "10 minute adventuring day" which has players blow their spells and daily use items, then retreat to recover it all with clerical spells and come back the next day. Plenty of campaigns have good time pressure to dissuade this, but my experience in classic megadungeon play is that this strategy is almost always the optimal one. And coming up with reasons to have a time crunch all the time is another burden for the referee, and can start to feel forced.
What helps this also is (and you may have noticed in my above rules) that I got no clerics in there. First, cleric advancement, both spells and turning, is very tied to levels in D&D, much moreso than fighters, who rely heavily on magical weapons, and magic-users, who rely on finding new scrolls and spellbooks to increase their repertoire. This makes them kind of hard to jam into the Gamma World framework without having to write new rules. To be honest, good riddance.
In general, high HP and low healing work much better for the HP as resource grind of old school dungeon-crawling than low HP and high healing, as is often house-ruled in or used outright in many slick, new, OSR adjacent games.
The cushion of HP gives way to more freewheeling, dungeon action, while the lack of healing means that all those fights have heavy consequence.
In conclusion
Play some Gamma World, it's fun! Despite some reservations I had when I first read it, it actually solves some of my nittier picks with D&D. You may just find it does the same for you, and want to bring some if its ideas with you.