I am sure this has already been stated, but I wanna have some writing this out :P
I think a majority of the player base, as of right now, prefers running Diadem and Relic because it is either easier than Alexander Savage and ThEX or because there is simply nothing else to do for them (and I am sure there are many other reasons). So, what if you left Diadem and Relic with base stats as powerful as raiding gear (ilvl 210), but allowed raiding gear to have set bonuses? Here are some examples I was thinking of the other day.
Restrictions: only Head, Chest, Hands, Legs, and Feet are available for set bonuses.
PLD
2 set: Spirits Within restores 100 TP upon use. Cooldown of Spirits within reduced to 15 seconds.
4 set: Cover now protects against Magic based damage, and adds an additional 20% damage reduction for incoming damage while activated.
5 set: While using Hallowed Ground turn into a Servant of Halone (cosmetic).
WAR
2 set: Berserk no longer pacifies after use.
4 set: Only 4 stacks of Wrath are needed now to use Wrath abilities. (Not applicable in Deliverance.)
5 set: While using Holmgang release the Inner Beast (cosmetic).
DRK
2 set: Increase Shadow Wall damage reduction to 40% and increase duration to 14 seconds.
4 set: Dark Arts no longer takes MP and no longer needs Dark Side.
5 set: While under the effect of Living Dead you channel the essence of Diabolos (cosmetic).
WHM
2 set: Regen spreads to near by allies every 7 seconds, players with the lowest HP will be the first jumps.
4 set: Medica 2 now refreshes any active Regen.
5 set: When Benediction is casted, a glimpse of A-Towa-Cant is revealed (cosmetic).
SCH
2 set: Dissipation now guarantees the next Aldoquium or Succor to Critically Hit.
4 set: Critical Heals on Succor now double the Barrier granted to that player.
5 set: Emergency Tactics now calls fourth an army of WAR Tonberries to protect you (cosmetic).
AST
2 set: Time Dilation's duration is increased to 20 seconds and cooldown decreased to 60 seconds.
4 set: Cooldown of Synastry reduced to 90 seconds. Player chooses the next card drawn after casting Synastry.
5 set: Reveals the Zodiac constellation Sagittarius while under the effect of Collective Unconscious.
DRG
2 set: Power Surge is no longer used by 1 jumping attack, but instead last the duration of 10 seconds.
4 set: Geirskogul potency increased to 300. Blood of the Dragon is no longer needed to use Fang and Claw or Wheeling Thrust.
5 set: When casting Dragonfire Dive, while under the affect of Power Surge, call fourth an army of Dragoons to pummel your foe (additional damage of 1 if you are attacking a dragon) (cosmetic).
MNK
2 set: Shoulder Tackle no longer Stuns, but instead grants +1 stack of Greased Lightning. One Ilm Puch now knocks the target back 30 yalms.
4 set: Using Tornado Kick grants Perfect Balance for 6 seconds.
5 set: Using Perfect Balance (skill) calls fourth Hamon Holyfist to pummel your competition (cosmetic).
SMN
2 set: Ruin III no longer costs MP when casting against a target under the effects of Bio, Bio II, and Miasma.
4 set: Enkindle cooldown is reduced to 120 seconds.
5 set: While under the effects of Dreadwyrm Stance, your egi takes on the form of the Elder Primal, Bahamut.
NIN, BLM, MCH, BRD
I have no idea how these classes work post level 50, so not even going to try.
I had fun writing this up and hope some of you had fun reading it ^_^ I am sure somethings would be completely broken while others would be well tuned.