I would do some unholy hybrid of DND5E, GURPS, AGE.
What to keep from D&D5E?
1. 20 levels. It's both familiar and iconic.
2. Proficiency bonus of +2 to +6 with expertise(×2 bonus) and ×3 bonus for some features/number of uses.
3. Tiers of play. Just re-tuned to 5 tier instead of 4: 1-4, 5-8, 9-12, 13-16, 17-20
4. Spell levels. Just reduce spell levels from 9 to 5. Plus cantrips as normal.
Full casters: 1st level spells at 1st level, 2nd level spells at 5th level, 3rd level spells at 9th level, 4th level spells at 13th level, 5th level spells at 17th level.
current spell level 1&2 are rebalanced into 1st level spells
current spell levels 3&4 are rebalanced into 2nd level spells
current spell levels 5&6 are rebalanced into 3rd level spells
current spell levels 7&8 are rebalanced into 4th level spells
current spell levels 9 are rebalanced into 5th level spells
some spells from current levels 2,4,6 and 8 might be buffed to be moved to new levels 2,3,4 or 5.
5. Extra attacks:
martials get extra attacks at levels 5,9,13 and 17. Total of 5 attacks per Attack action at 17th level
That is, you can "buy" extra attacks at those levels.
6. cantrips: rescaled for extra damage/effect at levels 5,9,13,17 instead of 5,11,17.
for other effects;
1. 3d6 instead of d20 as core roll die.
double damage for roll 5 over AC/Defense.
triple damage for roll 10 over AC/Defense.
2. Every ability/feature can be chosen by everyone.
Only limited/balanced by required character level, other ability/feature prerequirement, minimal ability score., race(species)
3. Everyone proficient with all weapons. Min STR required to use without penalties.
4. Everyone proficient with all armors. Min STR required to use without penalties.
5. ability scores are direct modifiers to rolls/DCs.
ability 0 is below average,
ability 1 is average,
ability 2 is above average,
ability 3 is maximum at character creation
ability 5 is maximum for standard humanoids without magic.