I think dungeons should be a little bit longer and harder but this will conflict with the casualism so i thought about a new system for dungeons :
Basically, the longer you get (by killing bosses), the harder the bosses will be (more hp, more damage, more abilities) and the better the reward would get(higher ilvl). The dungeons will also be divided in stages, so you still get a reward even if you didn't complete the dungeon (a little satchel or something).
Also once a week you get a big reward by finishing a whole dungeon(say a dungeon takes about 1 and a half hour to complete).
This idea came up while i was thinking how dungeons can be better, how the game can be better. Blizz stated that they won't add dungeons in patches anymore (at least for MoP) because they are only a "one-time-run" for gearing/preparing for raids, so i thought they might upgrade them a little bit.
This system is not entierly new, it's used in Challenge modes and scenarios (dividing the dungeon, say Stage1, Stage2)
TL;DR - New dungeons will reward you based on the effort put into it while still rewarding the ones that do not have time for completing them.