My first character ever has yet to reach level 50, but this sounds very similar to how RIFT Online deals with leveling after 50.
As long as I'm still getting soloable quests from NPCs after 50 that doesn't involve raids or PvP or dungeons, that's the only important thing to me.
"Soloable" = Nope, due to how rough and tough mobs are, plus the new formula for party exp there meaning you even grind 6 times faster than solo in a 5 person team, if you somehow could endure and survive mobbing. Grinding at least without a healer at this point is out of the question. Now, for those who have worked hard enough with it, heal pets do qualify as a healer... This is probably your best bet for solo grind, but this will then equal the 'grind 6 times slower' than a party of 5.
"Quests" = Nope, quests simply end at Master Levels, and can no longer be used at all for grinding. They only offer a small boost each day. By Master Levels, grinding begins officially.
"Raids" = True so far, as there are no raids. The Third Tier of Dark Whisper is expected to be our next Raid, and knowing how required such stuff becomes is pending that content.
"Dungeons" = Grinding 15-20 in the field will not work well with current content, but maybe the field tiers of Dark Whisper would fit for this. However, you can still grind in the first room of Osiris as if it was a field map and just reset it to respawn the mobs (which gets you to 20).
"PvP" = Yup, you're good here so far.