I've been at end game for a few weeks now. It's not bad, but it's certainly not nearly as polished an experience as the initial story leveling experience was.

1. Dungeon packs should drop 1 tome. Adjust final boss tomes for balance.

2. Should be able to bank Lvl 50 XP and use it for various things.
i. Gain rested XP for subjobs - 20-25xp bonus XP for 100 XP earnt @ 50.
ii. Small extra stat gain - like FFXI
iii. Use it to 'level equipment'. Like spiritbonding, except better - 1-2 point stat changes possible.

3. Low level dungeons should get end-game versions. Gear dropped can be recolours. Infantry/Battle Mage gear @ 50 = <3

4. Dungeons should reward based on supply and demand. i.e. The more players running a dungeon at once, the less it pays out. The less players running a dungeon, the more it pays out. DF tells you ahead of time how many tomes you get.

5. Extra jobs @ 50 should increase myth tome cap by 20-30 per job.