My ideas on rules for DnD and the like

Okay, this really is the half baked stuff that no-one in their right mind would want. Its just here for my own reference and as a slight exercise in ego stroking.

HD
I want monsters to be roughly measurable in HD in the way that 5e monsters really, really aren't. Its nice and easy, and elegant. The only problem is that its hard to scale up. Will have to think about more.
PCs won't gain one every level. Fighters and such get them quicker, wizards and such get them slower. Levels where you don't get new HD let you reroll you old ones, and keep the total if its higher than your previous total. Slower scaling.

Classes
There may only be four, like some hack-games? Fighter, Arcane, Divine, Specialist.
On the face of it, I like classes. But the ones in 5e are a bit bloated, I think its fair to say. And its annoying that I can't go level 3 Paladin level 2 Fighter because then I wouldn't have extra attack, which is stupendously good compared to what I would have. I think in that case, and cases like it, you should. Spellcaster multiclassers keep up slot progression. Why not fighters? Its easy enough to fix in 5e with a little house-rule, but I think I might mix it up even more stupidly than I should.
I think four base classes (maybe 5, unlikely 6) is enough, and then make everything a subclass of those basic four. Barbarians are just types of fighter really. Paladins really are just a type of fighter. Sorcerers and Wizards are really just types of Arcane. Etc. You can still distinguish them nicely between each other with subclass features, but they use the same basis, which hopefully helps balance too.
Subclasses also wouldn't have all that many new features, in that I think they'll mostly use the basic class features. For example, all fighters have maneuvers; paladins have their own special maneuvers, barbarians have their own special maneuvers, etc. I think they will be fueled by class specific dice. Fight Dice, Magic Dice, Faith Dice, etc. Paladin Smites are really just rolling Faith Dice when you hit a dude and channel the power of your CONVICTION THAT YOU ARE RIGHTEOUS AND THEY ARE NOT MUTHA BLASPHEMA
Nesting archetypes would be annoying though, not sure how to handle wizard subclasses like necromancer/illusionist. As I said, nested subclasses seem inelegant, Arcane having two subclass-esque parts also seems inelegant, having wizard, illusionist, necromancer be seperate also seems stupid to me (sorry Gary).

Ability scores
I really like these multi-ability score things (I know this guy didn't originate them either by their own admition, but its where I found out about them), but I really don't know how to implement them. Classic stats only exist in character creation? These extra stats sort of exist in that some abilities use two modifiers but no more than that? Have both and be really complex when one set will barely ever be used? Ditch one and hope the other can handle all the load? I'm not sure. One day I will figure it out I hope.

Movement/Chases?
Yo, grid based movement is kinda dope, in that it facilitates tactics without needing loads of cognitive load, like how I like to imagine 4e does it. But, not everyone has a grid, likes a grid, and grids make fights look static and dull.
Here's an idea. Speed is now a dice size. You also have a "stat" called position. You roll your speed against other creatures to try and improve your position against them. Closing into melee is now a roll, but if the other person is doing something else, its automatic, if the target is actively trying to keep you away, then its a roll off. Chases are similar. You want your position to be as good as possible to allow you to escape, but the other guys also want their position as good as possible to stop you.
Position also helps in combat. That's why fights in all the good movies move around so much. Each combatant is trying to improve their position. The fighter with the superior position is at an advantage.

Non-class things.
Tbh, even subclasses may end up as these. I imagine each person having a set of slots they fill with "cool things" like cool backstory bits, non-human races, subclasses, weird abilities like scrimshaw carving, alchemy, strongholds, destinies. They are extra things you can do. They differentiate characters, and in a way that I think is more realistic too. All the things you can do are not the result of your job. People are much more rounded than that. A lot of stuff might come from your class, but not everything.

Skills
They're nearly fine in 5e, but they're still stupid. Why are rogues and bards the only ones that get expertise? Except the worst fighters, and maybe a couple of others who get something like it.
I'll agree, 3es skills were overly complicated, but I think a lightly expanded list based off of 5es, with more gradiated skill levels (probably about 6, but no fewer than 4, not more than 8) would be cool, and differentiate between "I did this some in my backstory years ago but I still remember it roughly" and "I have trained and trained and trained. Fear me" Plus, it gives you something to train for, which leads us neatly to...

Downtime
I want this to be nice and robust. I want to plan shit, and be rewarded for it. I also want it to be simple enough that if you don't care for it, you don't need to bother all that much beyond "I want better skills, I will train." or "I want to talk to the duke, I will socialise." Maybe take some inspiration from blades in the dark. This is where the meet and potatoes of some of the Slots/subclasses may end up. Strongholds are in downtime for sure, and wizards need to research/copy spells, magic items need crafting, magic tattoos need inking, you get the picture.

Spells
I like Goblin Punches spells. I don't think I'd simplify them as much, but certainly simpler than 8 levels of spell.

Power Curve
Not as steep, not so high.

Abilities
Oooh, warlock invocations are super cool. All classes should have their version. Speaking of...

Warlocks
Not a class, a slot. Demons are way less picky than only choosing 0 level characters.

That's all I can think of. May add more as and when I get around to it.