Remain level 60
Have talent trees along the lines of WotLK.
51 talents at level 60 with those trees would make for some varied builds. Could spend all 51 in one tree for that last skill or 31/20 or 41/10. Dont necessarily keep all the skills given in BC and WotLK though.
Add what was learned from BC and WotLK with the games itemization. This would make more builds viable.
Jewels and jewelcrafting
Glyphs
Northrend added to the game but make it one endgame zone instead of multiple. Level 60 raid and 5 man instance in this zone. Could use existing assets from wotlk to make this zone easy to build.
Outland added to the game but make it one endgame zone instead of multiple. Level 60 raid and 5 man instance in this zone. Could use existing assets from BC to make this zone easy to build.
Hyjal zone added to game. Could make this a leveling zone to fill in a gap that is needed at some level bracket.
Emerald dream added to game as a level 60 raid and mid level 5 man dungeon.
Kharazan added to game as a 10 man raid
ZG and AQ20 changed to 10 man raids
All 40 man raids changed to 25 man
All raids accessible from beginning of game
Naxx, icecrown, black temple, emerald dream all similar difficulty but with different items people would want from each
Raid encounters involving every resistance
Fire: MC, BWL
Frost: Naxx, Icecrown
Shadow: Naxx, Icecrown, BT
Nature: AQ, Emerald dream
Arcane: Kharazan, Black Temple
Every class able to receive a legendary weapon like from Legion expansion. This weapon would be obtained by getting a specific item from each of the end raids, naxx, icecrown, black temple, emerald dream.
All endgame 5 man dungeons given drops that give high geared players incentive to return to them for the life of the game (similar to righteous orbs in strat)