Just thinking, what if instead of bumping up the level cap, you just made old content from previous expansions bump up to higher rewards?
Example: Pretend its 2013 and 5.4 Drops and the level cap is 90. All the really old content is worthless, all the closer old content is less relevant. MoP had 3 raid tiers.
Why not just set three difficulties for raids: LFR, Timewalking and Standard.
Timewalking raids are all previous raids bumped up to a current expansion/cycle's relevance.
So instead of all these different difficulties you have:
SoO (Heroic equivalent difficulty in 5.4)
ToT (Flex equivalent difficulty in 5.4)
Terrace, MSV, HoF, Timewalking (Normal equivalent difficulty in 5.4)
And in 5.2 the equivalent would be
ToT
Terrace, MSV, HoF
Timewalking
And then LFR is for all raids, and awards a level of gear equivalent to Heroic dungeons but slightly stronger, IE, better optimized, more epics, tier set bonuses.
Under this system:
1. Old content stays relevant
2. Old content can be played as old or new: Run Tier 6 Sunwell or Tier 14 Sunwell in 5.4.
3. LFR level players can see the story and play more casually, and progress their character.
4. No need to design so many raids around so many difficulties just for them to be replaced and forgotten.
5. No need to run the same raid one million times across all the difficulties, meaning you have actual refreshing variety in your raid progress.
This can apply to dungeons, scenarios, events such as the escalation patch as well. Only issue I see is with world bosses but I'm not sure those are so make or break. Could probably make it apply to old world content in some way.
Dungeon example in 5.4:
MoP Heroics = Timewalking Heroics
Timewalking Dungeons = Leveling MoP Dungeons
Only time the Dungeon example wouldn't apply is when an expansion is fresh, so current heroics are preferred.
Overall, in the Raid example, you can now progress at 60 this way:
Standard Mop/Timewalking Dungeons > High Level Quest Gear, Crafted, etc. to bump up to your Heroic dungeon requirements > MoP Heroics and LFR > Timewalking and Tier 14 > Tier 15 ToT > SoO
Then maybe for completing an expansions Pathfinder, you are incrementally awarded more power to your character such as talent points, passives, or whatever, in addition. So go quest in Northrend when Wrath drops if you want to feel like a more powerful 60 than you did in Outland.
I would say the only caveat is that experiencing a Timewalking raid wouldn't be just like experiencing it as current content; To save development time you would simply have to tune numbers or raid wide buffs which would change approaches for sure, but not in any way that would really have some disastrous effect.