I will just list the pre-requisite steps required for a fresh character to jump directly into the final raid tier of each expansion. Reach your own conclusions from this.
BC:
- Grind heroics for badges to buy the catchup gear.
- Enchant and gem your gear for specific caps (defense cap, hit cap)
Wrath:
- Grind heroics for badges to buy the catchup gear.
- Grind rep for the shoulder-slot enchants.
- Enchant and gem your gear for specific caps (defense cap, hit cap)
Cata:
- Grind (the same 3) heroics and LFR for pre-raid gear and badges.
- Grind rep for the shoulder-slot enchants.
- Enchant and gem your gear for specific caps (defense cap, hit cap)
- Reforge your gear for specific caps.
MoP:
- Grind heroics, LFR, and scenarios for pre-raid gear and badges.
- Complete the TIME-GATED legendary cloak questline.
- Enchant and gem your gear for specific caps (defense cap, hit cap)
- Reforge your gear for specific caps.
WoD:
- Complete silver on the proving grounds to be able to queue for heroics.
- Grind heroics, LFR, and apexis dailies for pre-raid gear.
- Complete the TIME-GATED legendary ring questline.
- Do the LONG quest chain for Tanaan Jungle which is TIME-GATED behind a shipyard mission.
Legion:
- Grind heroics, LFR, Invasions, and M+ for pre-raid gear.
- Enchant and gem your gear.
- Unlock Broken Shore so you can unlock Argus.
- Complete the LONG Argus storyline so you can unlock the crucible.
- Grind AP to be able to gain the +15 ilvls from the crucible. (Repeat for each spec played).
- Complete the LFR storyline for your first free legendary.
- Complete your class hall’s TIME-GATED research to be able to wear 2 legendaries.
- Grind your 2nd RNG-based legendary drop.
BFA:
- Grind heroics, LFR, random gear token drops, and M+ for pre-raid gear.
- Enchant and gem your gear.
- Complete the first part of the Nazjatar storyline to unlock essences.
- Complete the legendary cloak questline.
- Grind for AP to unlock your Azerite armor slots.
- Grind for AP to unlock your essence slots.
- Grind the Time-Gated 8.2 rep for essences.
- Grind the Time-Gated M+ and/or rated pvp essences (repeat grind for additional played roles).
- Grind the Time-Gated 8.3 dailies and weeklies for essences.
- Grind the Time-Gated weekly visions for your cloak upgrades.
- Complete your Time-Gated vision research to even be able to go deeper into the visions.