I've fallen out with RAW 5E for a number of reasons - but I am planning to have a go with AIME... currently running some 1E though and enjoying that a lot more.
If I was to go back to running 5E, these would be my houserules:
- no multiclassing unless the story called for it, and even then just one switch and you can NEVER go back
- Dwarfs and Halflings are not arcane casters
- (the opposite of most here) alignment rules to be enforced akin to 1E
- fantasy racism exists and is rife
- the party is a team - you must all agree beforehand to play similar alignments and stick to them.
- All loot is party treasure. You may steal from a fellow party member but it is pointless, because you'll be stealing from party treasure to put it back into party treasure.
- nothing from UA, if it's not in an official hardback 5E publication it does not exist
I need hit points to be a more precious commodity. I'm toying with more gritty healing - for recovery of class abilities/spells then the standard 1hr/24hr applies, for recovery of Hit Dice/Hitpoints then 1 day/7 days. Also the following changes - a fighter's second wind recovers HD during a short rest, not hit points during a combat. If you are down to 0hp during a fight you will remain unconscious for the duration of the fight, no miraculously bouncing back up at 1hp in the middle of combat.
Magic items will have to be Identified before attunement/use is possible.
Poison effects will be much more lethal and long lasting. Fail the save and you will likely be incapacitated for days. Make the save and (depending on the creature), you may still be out of action for a few hours.
I agree in part with dwarves and halflings. It's possible, but has restrictions and disadvantages.
Fantasy racism, yes.
Agree with the team, and identifying magic items.
For the poison and hit points, I use a different system that addresses both. Poison, disease, ability damage, life drain, and injuries all use the exhaustion track, combined with the death saving throw mechanic. In most cases you make one save daily, and you can get better or worse depending on your saves. So the effects last for at least 3 days, usually longer, and completely separates the major effects from hit points.
I do heal fully at the end of a long rest, but your Hit Dice are spent automatically outside of combat. You roll your hit dice at the beginning of the day, and top off your hit points following each combat. It's basically a pool that represents what you can handle for a full day, which is more than you can handle in a dingle encounter. It works really well. Magical healing and a bard's song of rest, along with the Healer feat heals points first.
Also, you can't just stabilize a creature. Applying a healing kit allows them to add their Constitution bonus to their death save, but they still need to make it through 3 death saves, and they suffer a level of exhaustion for each failed save which is recovered normally. Lesser or greater restoration doesn't cure disease, neutralize poison, or heal injuries, although it can help. I'm happy to share the rules, but they are sort of interconnected - healing, magical healing, injuries and such, etc. Also some combat stuff - injuries can come from critical hits, falling, traps, etc.