upgrade heirlooms to 110 except weapons

NPC Scan addon for rare mob find

lvl 45: go to darkshore / felwood

stay in classic until lvl 60 and get flying

at 60 go to Borean Thundra, at 67 to icecrown or scholazr basin

70: learn faster flying

Do NOT queue for Cataclysm DUNGEONS

80: hyjal und jade forest to 90

at 90: ask mage to portal to stormshield (skip scenario)

90-98: fly around and get treasures and do bonus objectives
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Start with both Frostfire Ridge and Shadowmoon Valley, then move to Gorgrond-Thalador-Peaks of Arak. By the end of Arak you should be 98.
Remember, that treasures in that zone require minimum level of that zone. So you can't collect treasures in Arak unless you are 96+. In other words, there's no point in going to collect in other zones while you are too low-lvl for them.

98: get artefact in legion
azuna and stormheim questing IF there is no invasion up. do those as priority

110: blue tokens from argus on main char: if so, buy gear. few world quests (go to kadgar to unlock)