Further Beyond (5e Variant Rule)
From D&D Wiki
As many great minds have said, big numbers are cool. However, Fifth Edition is relatively conservative is this regard, and with good reason. Players are heroes, not gods, and monsters, even the most dangerous, are slayable. But in some universes, this simply isn’t the case. This variant rule is best used as a supplement to the Level 40 Expansion or a similar ruleset for core classes, but is primarily meant to facilitate infinite level scaling with additional homebrew. Additionally, mythic monsters or player equivalent CR are a must for any campaign above 20th level.
A creature’s ability modifier is equal to (their ability score - 10)/2, rounded down. As normal, you can't increase an ability score above 20 using Ability Score Improvements. At 20th level, this limit increases to 30, at 30th this limit increases to 40, at 40th this limit increases to 50th, and at 50th this limit increases to 60. Ability score increases are gained at a minimum of once every 4 levels. This pattern continues indefinitely.
A creature's proficiency bonus is equal to ((their level-1)/4)+2.
If a creature can not level any further in their class, they must multiclass into any class that they meet the requirements of.