I'm starting a new D&D campaign next week (just did our Session Zero last week), and part of that is finally gathering together what new houserules I want to apply to my games going forward.
A Whole Lot Of Things Are Flexible
First of all, everything in the Class Features UA is allowed. This is mostly just a lot of things giving players a little more flexibility - letting them swap out spells and other abilities for ones of equal level, etc. It's all solid.
Second, casting classes can use any mental stat you want. Want a Wizard that casts with Cha rather than Int? Go for it. Want a wise Bard? Cool. A smart Warlock? Excellent. The mental stats are all more or less interchangeable with each other, and the assignment of stats to classes (and the attendant ease or difficulty of multiclassing as a result) is 100% flavor-based, not balanced-based. (Other classes that use mental stats for abilities are similarly flexible - Monks can use Int or Cha for their AC, abilities, etc.)
Third, damage types are interchangeable. Want a lightning-themed wizard, and annoyed at the relative paucity of lightning damage compared to fire? That's fine, take any damaging spell and swap it to lightning damage. In general, fire/ice/lightning/thunder/poison are all equivalent and can be swapped, and radiant/necrotic/psychic/force are also equivalent and swappable. Swapping between the groups is a minor balance concern; it's possible, but requires some manual tweaking (usually just adjusting the damage dice).
Finally and most generally, anything can be reflavored. The mechanical skeleton - the actual rules and numbers - are important to maintain, but if you wanna make it present differently, fuckin' go wild. Nothing is canonical, please take whatever rules can best be bent to your concept and reuse them.
D&D's default flavor can work for a lot of people, but there's so much more you can do when you open it up a little.
Unearthed Arcana and Decent Homebrew Is Fine
Stuff from Unearthed Arcana is often quite fun and reasonable; unless something's been rewritten since, consider it usable.
Homebrew stuff is also generally fine; I've been doing homebrew design on a personal level for 20 years, and am pretty good at eyeing balance and tweaking when necessary. We can always adjust on the fly if it turns out too weak or strong.
There's just a lot of great homebrew stuff! I particularly like a lot of stuff from Mage Hand Press, like the Warmage (cantrip-focused fighter) and the Craftsman (item-crafting warrior). And 5e is far easier to balance for than previous editions, so a lot of it is pretty high quality.
Mundane Equipment Is In The Wagon
You can track your personal equipment normally; whatever you want to make sure you have on you.
Otherwise, all your mundane equipment is considered to be in the Wagon (/Backpack/Home Base/whatever). Everyone contributes money to the Wagon's pool; whenever you need any equipment, you just get it from the Wagon and deduct the cost from its total.
If the item is an obvious thing you'd definitely have packed (basically, anything from one of the premade item packs they start characters with), it's definitely in the Wagon.
Otherwise, you can provide a justification for why you'd have definitely bought the item sometime in the past. Based on the plausibility of the excuse, I'll determine a DC, and you make an Int or Wis check. On success, your character did indeed have the foresight to purchase that, and it's in the wagon. On a failure of 5 or less, you didn't quite get what you needed, but there's something close. On a worse failure, you didn't actually pack that, and you can't retry for it until you have a chance to restock.
Jeezus, tracking mundane items is a fuckin' slog. In general the prices become utterly insignificant past 3rd level or so, so it becomes purely a game of "did someone read the equipment chapter and just transcribe a bunch of it to their character sheet". Just assume people have what they need, with a little bit of game-ness to make it fun and reasonable.
Fast And Slow Initiative
Described in more detail in another blog post, but in general, rather than rolling initiative and establishing a battle order for the rest of the combat, each round players decide whether they're going "fast" or "slow". All the fast players take their turn, in whatever order they work out amongst themselves, then all the fast monsters, then the slow players, then the slow monsters. Next round you make the fast/slow choice again.
Going fast is a normal round. Going slow lets you impose advantage or disadvantage to a single roll that anyone makes during your turn (as you have taken some time to watch the unfolding situation and take a more optimal action). If you have initiative bonuses from some feature, the linked blogpost describes how to translate that into fast/slow bonuses.
Already explained in the blog post. Initiative is just a fuckin' slog, man. Fast/slow is fast, dynamic, and encourages strategy and paying attention.
Resting Is Harder
Short rests are left as they are.
Long rests now restore all Stamina (see next section for Stamina/Health details), regain one hit die, recharge anything that recharges on short rest, regain spell slots of combined level equal to your character level, and recharges one ability that would normally recharge on a long rest (besides "spell slots").
A new Rest downtime activity (spend ~a week resting or doing light activity; can be combined with other downtime activities if they're not physically stressful) restores all Health and Stamina, all hit dice, and recharges all abilities that would restore on short or long rest, including spell slots.
The D&D overall ability economy, balancing classes focusing on constant and short-rest abilities (like the Fighter, Monk, and Warlock) against classes focusing on long-rest abilities (like the Barbarian, or most casters), is predicated on the assumption of 3-5 encounters between long rests, with a short rest every 1-2 encounters. This is intended to allow short-rest classes to exhaust their relatively small resource pool in approximately every combat, while long-rest classes have to ration themselves across several battles.
In practice, however, in the ~20 years I've been playing D&D, this is almost never the case. Instead, it's almost always 1-2 encounters per day; or when it looks like it'll be more, the players usually contrive to long-rest in the middle. This ends up unbalancing the classes in a frustrating way, giving casters especially more power than intended.
Splitting rests into three categories as shown should let me continue to play D&D as I always have, while bringing long-rest classes back into reasonable balance with the other classes. This does rely on inserting downtime potential between distinct "adventures", but that's something a DM should be doing anyway; downtime is fun, and helps alleviate the "peasants to gods in 30 days" leveling-speed dynamic that can accidentally happen otherwise. I like designing explicit "missions" into the structure of my game anyway, so it should slot in nicely.
I am going to be playing with the exact balance of the "recharge one Long Rest ability" thing, especially for casters, so don't take that as set in stone currently.
Health/Stamina Instead Of HP
Rather than a single pool of HP, players have separate pools of Health and Stamina. Stamina is lost first when a player takes damage; when it's all gone, excess damage goes to Health. (Same as the existing Temp HP vs HP mechanic; Temp HP of course also continues to exist, and is lost before Stamina.) Players are unconscious/dying only when both their Health and Stamina reach 0.
Stamina represents your ability to turn attacks aside at the last moment, or suffer only bruises or scratches, etc. These are things that, with a breather, you can recover from and get back to fighting fit. Health is your literal physical health; losing it means you're actually taking injuries.
All healing effects - spells, rests, etc - restore Stamina only. There is no way to recover Health while out adventuring (normally); it can only be restored by bedrest (the Rest downtime activity from the previous section).
At first level, if a player has a 1dX hit die, they now have 4 Health and (X-4) Stamina. At later levels, they gain 1d4 Health, and a flat (X-4) Stamina. Any other effects that raise max HP (Con bonus, feats, etc) raise Stamina.
D&D's binary health system, where you fight at 100% full strength from max HP to 1 HP, then immediately fall to the ground and start dying at 0, has always been silly. It also means that, sometimes, the most efficient move is to let a player that's almost out of HP tank a big hit and drop unconscious, then heal them up a tiny bit to get them back fighting. That's ridiculous!
I've always liked the idea of systems representing graded damage levels, but they've all been bad - either they're too fiddly and require constant adjustment of numbers, or they cause death spirals when things start to go bad, or both.
This is an attempt to give a tiny bit of the feeling of graded damage, without significantly affecting balance, particularly within a battle. As long as you stay fairly topped up in HP, there's no change. If you get down to your last few HP, nothing changes stat-wise, but after the combat's over and you heal back up, those HP stay lost until the adventure's over and you can heal up properly. So there's incentive both to keep yourself topped up when possible, and to break between adventures to rest and heal.
Note that Health is intentionally meant to be a relatively small fraction of your overall HP, especially for front-line fighters with a d10 or d12 hit die and good Con. (A 10 Con Wizard will have about half their total HP as Health, but a 14 Con Barbarian will only have about 1/6 of their total as Health. The Barb can take a lot of beating before they actually suffer an injury that'll stick around.) Also, especially for large hit-die classes, Stamina by itself is equal or larger than HP normally would be, so it shouldn't even hurt them balance-wise anyway; if you play exactly as you would with normal HP, you'll stay in Stamina anyway and never suffer a wound.
Finally, this allows you to still reasonably roll your hit die to determine your HP growth as you level, which some people really enjoy, without allowing someone to get absolutely crippled by a low roll, or even a few low rolls in a row! The diff between min and max roll is just 3 Health each time, which is a fairly small part of your total.