Hmm, where to start;
1. Races no longer provide ability score bonuses(penalties). At least not base ones.
2. Ability scores and modifiers go 1-for-1. I.E. 10 is +0, 11(+1), 9(-1), etc...
3. Point buy: 16pt pool
9(-1): 0 pts
10(+0): 1 pt
11(+1): 2 pts
12(+2): 3 pts
13(+3): 5 pts
14(+4): 8 pts
4. Separate pool for ASI's, Combat feats and exploration/social feats.
5. Sub-classes for all classes at 1st level.
6. Add 10th level spells for full casters at 19th level.
7. Remove simple/martial weapon split. All weapons are at martial level. All are proficient with all weapons.
Fighting style(s), Extra attack(s), martial maneuvers, investment in STR/DEX will differentiate martial from non martial character well enough.
8. Remove medium armor. Add buckler with light armor for +1 AC. Normal shield comes with heavy armor proficiency.
9. HDs healing replaced by 4E style of healing surges(25% of max HPs), number per long rest depends on class and extra feat(ures).
10. Short rests removed or made really short. I.E. 1 minute with 2 or 3 limit before long rest.
11. Long rest 10hrs long with 8hrs sleep. Elves can wiggle down this to 6hrs(4+2).
12. Races(lineages) now only add "genetic" abilities.
I.E. ALL elves have keen senses. so they could have proficiency+expertise in Perceptions.
Humans are versatile and fast learners, so they get +2 skill proficiency of choice.
Half-elves are in between so they get proficiency in Perception, plus one skill of choice or expertise in Perception(to describe stronger elven blood).
Low-light vision/darkvision is also genetic, so is base speed and forms of movement in addition to walking(swim, burrow, climb, fly, teleport).
So are resistances and immunities to various type of damage or spells.
Dwarves are tough so they get extra HPs, resistance to poison and disease or similar things.
Culture and/or background can add most of bonus skills to characters or maybe armor proficiency or fighting style.
13. add 2/3rd caster: 1st level spells at 1st level(but less spell slots than full caster at 1st level),
2nd level spells at 4th level,
3rd at 7th,
4th at 10th,
5th at 13th,
6th at 16th,
7th at 19th.
14. completely try to avoid getting brand new abilities at higher levels.
Base level of all abilities should be acquired by levels 10 or 11.
After that it should be just improvement on existing abilities and/or more usage per rest.
15. you can only get 4 long rests outside "relative" safe place. After that you need extended rest. 3 nights in a row at a safe "haven".
5th long rest in a row(and after) only recharges HPs, up to half healing surges and exhaustion cannot be improved above 2nd level if any.
no other "per long rest" abilities are recharged and you get only one short rest to use between long rests.
think of this as "weekday/weekend" mechanics.
16. return of "max dex modifier" for every type of armor. That way we can avoid having least amount of dex or max amount of dex characters.
I.E. padded armor could have max dex of +6 and fullplate of +0.
Light armor:
padded: 11 AC, max dex +6
leather: 12 AC, max dex +5
studded leather: 13 AC, max dex +4
hide: 14 AC, max dex +3
heavy armor:
scale: 14 AC, max dex +4, disadvantage on stealth
breastplate: 15 AC, max dex +3, disadvantage on stealth
chainmail: 16 AC, max dex +2, disadvantage on stealth
halfplate: 17 AC, max dex +1, disadvantage on stealth
plate: 18 AC, max dex +0, disadvantage on stealth
17. Multiclassing can only be done with 2 classes, and both need to be within one level of each other. No cheesy dips.