Honestly what would be a fun idea to allow for challenge to stay in the game is to create a system in which you can lock the il cap and your rewards are influence by your ilevel.
Let's use alexander for an example. In Normal Mode let's just say they decide no loot drops.
**To keep it simple I"ll say the entry I level is 110 and the i level cap is 150 (raid drops 140 that can be upgrade to 150)
For the Hard mode it would work like the following:
-Item Level=Loot Drop
Unlimited=2 chest which can have gear for any job
130=3 chest drops (may drop materials for unique vanity)
120=4 chest drops (may drop a unique minion)
110=5 chest drops (all players gain a unique token which can be used to purchase special items)
And pretty much players could impose their own difficulty into certain dungeons but at the same time be offered new and unique rewards for them. So while some players can say "I just want to beat this" others may want to go that extra mile and challenge themselves for additional loot and to impose this challenge their IL would be limited.
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This could also be reflected with 4 man dungeon:
Unlimited= il130 gear drop
110= gear drops + vanity materials
100= gear drops + minion + vanity materials
90= gear drops + minion + vanity materials + special token to exchange for unique items
Honestly I believe SE could exponentially increase the weight of their content by allowing players to impose their own difficulty while also giving them more rewards from the treasure pool. It would give players a way to hone their skills and take on dungeons that would otherwise be easy given the inflated item level player's can easily achieve in a months time.