I agree with above posters that THE only thing to do once you clear inferno, which lets face it EVERYONE can now, is farm Paragon in A3 and MP10 Ubers. However, making the content/difficulty OPTIONAL is a big deal for the casual players.
Some form of Horde mode is needed and a bump to MP15 would be a good start. This Horde mode (endless waves of increasingly difficult enemies, for those not familiar with Gears style Horde mode) with a ladder implemented for score tracking would be awesome I feel. Having a Solo, Duo, Trio, and Group Ladder system to track the Levels/Points would be cool, and every 5 levels or so the Act mini bosses could spawn in alone with mobs, and every 10 levels the Act Boss would spawn in. Once level 40 is defeated (I.E. Diablo) every 5 levels could spawn in somethign different. For Example:
Level 5 - Skeleton King
Level 10 - Butcher
Level 15 - Magda
Level 20 - Zolten Kulle
Level 25 - Seigebreaker
Level 30 - Azmodan
Level 35 - Rakanoth
Lelev 40 - Diablo
Level 45 - Skeleton King + Butcher
Level 50 - Magda + Shatterbone
Level 55 - Seigbreaker + Ghom
Level 60 - Izual + Diablo
Level 65 - Uber Skeleton King + Uber Butcher
Level 70 - Uber Magda + Uber Rakanoth
Level 75 - Uber Ghom + Uber Zolten Kulle
Level 80 - Uber Seigbreaker + Uber Izual
Level 85 - Skeleton King, Butcher, Madga
Level 90 - Zolten Kulle, Seigbreaker, Iskatu
Level 95 - Rakanoth, Ghom, Izual
Level 100 - Belial, Azmodan, Diablo
Etc. You get the idea.
The rewards could be somethign new eventually but for now Every 5 levels could drop:
1 Demonic Essense and a 6 Prop Rare up to Level 40
2 Demonic Essense, 1 6 Prop Rare, 1 Brimstone, an Organ Chance(on Ubers) up to Level 80
3 Demonic Essense, 1 6 Prop Rare, 1 Brimstone, 1 Legendary up to Level 95
4 Demonic Essense, 1 6 Prop Rare, 1 Brimstone. 1 Legendary, 1 Random Radiant Star Gem, 1 Set Piece
Just my thoughts of something insnanely crazy and "neverending" rewards could be different obviously, but then if you implemented a Ladder or scorboard tracker that would reset every month people would farm that !@#$ like crazy to try and be tops. We love competing against one another.
Just an idea I had real quick while reading through the forums. Its pretty rough, but its a concept that I feel the devs should look into. Just if you do, PLEASE GOD MAKE IT HARD!!!! It would be optional, after all.