What is probably going to happen is that in the expansion, the new level 50 zone/mobs/quests will be tuned to players in i50-55 gear, and said quests will give out items of significantly higher item levels. By the time you are on somewhere between level 55-60 quests/mobs the gear will probably match coil 3 endgame stuff, and then the level 60 entry-level dungeons will most likely be an improvement over Coil 3.

The reason I think it will be done this way is that it will provide a smooth transition from level 50 to level 51 and beyond. Then, At some point the quest gear will match or be an improvement upon Coil 3 providing the gear reset that we get every major patch.