A wise Pandaran once said to me “Slow down…”
Most levelling Challenges seem to have a focus on speed &/or difficulty and can be over complicated with various rules sets.
The Completionist Levelling Challenge is all about being thorough. The rules are really simple. You can play any class & any race on any server. Your are NOT allowed to use heirlooms & you are NOT allowed to be boosted. Once you reach level to 60 (or level 25 if attempting after the release of Shadowlands), you should lock your characters XP gains until you have completed the following…
Classic: 1-60 (1-25 after Shdaowlands)
- Fully explored all classic zones, so that you have the explorer achievement for each.
- Completed the majority of quests within all classic zones, earning the quest achievement for each zone.
- Complete all classic dungeons, so that you have the achievement for each.
- Optional - Earn Exalted reputation with all classic factions (currently still in game). There are two exceptions to this… 1) This does not include raid specific reputation grinds. 2) reputations associated with “Ambassador of the Alliance/Horde” achievement can be worked on throughout your journey as a whole, rather than a specific/single expansion.
That’s it. Once the above has been achieved simply unlock your XP, level onto the next mile stone (TBC), re-lock XP gains & repeat the process, earning explorer, questing, dungeon (and reputation achievements) as you level throughout each expansion as listed.
The Burning Crusade: 60-70 (25-27 after Shadowlands)
Wrath of the Lich King: 70-80 (27-30 after Shadowlands)
Cataclysm: 80-85 (30-32 after Shadowlands)
Mists of Pandaria: 85-90 (32-35 after Shadowlands)
Warlords of Draenor: 90-100 (35-40 after Shadowlands)
Legion: 100-110 (40-45 after Shadownlands)
Battle for Azeroth: 110-120 (45-50 after Shadowlands)
Again, the goal of this challenge is not to be difficult or super fast, but to be thorough and to experience more of the game rather than skipping over the majority of it like most levelling challenges. 
