TerraDave
5ever, or until 2024
Taking a bunch of time to learn a proficiency or language is already in the DMG, along with some other additional downtime activities. You could add skills and I guess spells, for a cost. I would not do expertise or mastery, could really break the game.
You could also do a more general training rule. E.g. 6 months and 100 gp per level for each level, or a "wine, women, and song" rule where you can spend gold for XP over time, a variation on the 1E rule where you got XP for finding gold.
You could also do a more general training rule. E.g. 6 months and 100 gp per level for each level, or a "wine, women, and song" rule where you can spend gold for XP over time, a variation on the 1E rule where you got XP for finding gold.