So the 5e rest system never really appealed to me. Here are my changes.

(Unless noted below all aspects remain the same for Short and Long Rests)

Short Rest

  • Reduced in length to 10 minutes.

Long Rest

  • Now requires 3 hours of light activity and 3 hours of no activity.
  • No automatic health gain to maximum.
  • Regain 1/2 of total hit dice.

(Added the following new mechanic)

Full Rest 

  • 1+ days of light activity based on the general condition of the party.
    • Condition determined by the DM, but the party is encouraged to logically sway that conclusion.
    • "Light Activity" is considered "downtime". Roleplay in a town, staying at a waystop, relaxing at home, etc.
    • "Travel" is not considered light activity unless it is under someone else's power. IE, in a wagon.
    • Some thing like hunting or camping might or might not be light activity based on the circumstances.
  • All spell slots regained.
  • All hit points healed.
  • All Hit Dice regained.
  • 1 Hit Dice added as temporary hit points to reflect a "well-rested state".
  • (Certain magical items and abilities might now take a Full Rest to recharge.)