its rotation no later than level 40

Possibly a bit early, and arguably off topic, but IMO every spec of every class should have all abilities/spells that are core to its rotation no later than level 40.
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Beyond 40, start adding various offensive and defensive cooldowns.

Any spell or ability that is considered core to a specs rotation, single target or aoe needs to be in that spellbook at a reasonable level. It is a huge oversight to leave things such as Soulswap out of an Affliction Warlocks book until 79, or to leave Rogues without any AoE AT ALL until level 66!