I guess some people wondered which features got added to MU Online with each new season. Me too, so I did some research until Season 6 some time ago. Here are my notes:

Season 1:

  • Crywolf event
  • Castle Siege event
  • Fenrir pet
  • Ancient Items (which drop in Land of Trials and Chaos Castle)
  • Lahap (jewel packing/unpacking)

Season 2:

  • Kanturu Event, Gemstones, Jewel of Harmony
  • Aida Map
  • SD for PvP
  • SD and Complex Potions
  • Level 380 Items (Bone Blade etc.)

Season 3:

  • Master character classes, skill tree and 3rd wings
  • New ancient items (Agnis etc.)
  • New Maps (Barracks, Refuge, Swamp of Calmess, Santa Village)

Season 4:

  • La Cleon Map
  • Summoner character class
  • Socket system with items, NPCs and level 1 spheres
  • Duel system with new duel map

Season 5:

  • Gens system
  • Fortress Imperial Event
  • Double Goer Event
  • +15 Items
  • Beginner Quests from Elf Soldier

Season 6:

  • Rage Fighter character class
  • Small wings
  • Inventory and Vault extensions
  • Official MU Helper
  • Additional jewels can be (un)packed by Lahap
  • Reworked master skill system

I hope you have a use for this information. If you think something is missing or wrong, please leave a comment. Thank you :)