I would like to see the heroic/regular difficulty mentality go away entirely.
Instead, move to a system by which difficulty is scaled by the number of people.
The normal dungeon group size would default to 10 people, like the classic level 60 dungeons started out. Encounters could be designed primarily around one tank and one healer, to drastically shorten queue's. The 'heroic' version of any dungeon would simply cap the group at 5 people.
I would like to see the same thing happen with raiding as well- normal raids of 20/40 and heroic raids of 15/30.
I also think a marginally larger default raid size would be a godsend, as we can't take one of every class in a ten-man, or one of every spec in a twenty-five man.
Further, the developers would only need to develop and tune one version of each dungeon, and two versions of each raid (and even then, the raids would be MUCH easier to scale). Further still, a 15-man minimum raid size would allow much more dynamic fights without needing gimmicks to make up for a missing tank or healer.
-More bosses, rather than more versions of bosses.
-Better scaling in raids.
-More dynamic battles and encounter designs.
-Faster dungeon queues for normal dungeons.
-Allowance for more specs/classes in a given dungeon/raid.
The biggest downside I could foresee would be the guild drama resulting from a guild progressing from normal raiding to heroic. I think this would largely be diminished by a more consistent content schedule arising from reduction in developmental effort per encounter.
The other potential downside would be the logistics of a 40-man raid. I am just guessing here, but I have to imagine the number of 25-man normal guilds has to be pretty low, as would the number of 40-man normal guilds. Of course, this could be alleviated by one additional raid tier, maybe called 'LFR', which used the 40-man size. 30-man could then be reserved for 'normals' and the current 25-man raid would be 'heroic'.