There is little to do in terms of solo content for endgame besides the beastmen tribe quests.
What I suggest is solo trials fights that could take the form of the level 30 and 50 class/job quest fight but be reworked to suit level 50 players.
It could be worked into a Wandering Minstrel quest seeing as how he played a part in giving the player access to hard mode of Ultima.
For example.
The quests grants access to a solo trials tab in the duty finder menu.
Upon starting the quest there will be a cutscene.
(Wandering Minstrel) Ah ,<player name>, the ballad of your victory over the Ultima Weapon is a success. Pray tell me more of such victories. After that the cutscene ends you are prompt to talk to him again and then a box like the one from the achievement npc appears with the names of the level 30 class quest and the level 50 job quest your character have done so far.
Let's take Pugilist for example. Once you select Return of the Holyfist your character will proform the talking emote followed by the Wandering Minstrel saying.
(Wandering Minstrel)So you helped Hamon Holyfist save the Pugilist guild by defeating Weggfarr Wideaxe. Yes, I can make an brilliant ballad. *fade out & then in* Now then wanna hear it. Then you get solo trials tab and it tells you to queue it up.
Used the Duty Finder info tab as a template.
As for the fight. The fight will be a different then the level 30 one.
It'll start with the cutscene where Holyfist sqaure off against Weggfarr, after Holyfist body blows Weggfarr. Instead of using the Pluto substance he releases a paralyzing mist that affects everyone but Weggfarr. After that he charges at you and the fight starts.
You start with a potent paralysis debuff that can't be removed until you get him to 85% and an cutscene starts with Rurukuta rushing over to you to give you the antidote but is knocked out by Weggfarr. Once you get him between 40%-50% he'll use an enhanced version of Barbaric Surge followed by repeatedly using Iron Tempest
for 45 secs. And when he is down to 10% he'll use an all or nothing attack instantly hitting you for half you current health you have when he does the move.
And here's the image of the job part.
The rewards itself will be job specific minus the new tomestones that'll drop with it.
For example.
1. Job specific accessories.
2. Job specific housing recipes. IE. An untradable interactive Sparring dummy for Pugilist and Monks to use
that increases critical hit rate by 10% once placed on you housing plot.
3. Special item needed for job specific housing recipe.
4. A key item that could be use in the customization stat part quest of the relic upgrade quest line.