If the difficult solo bosses were only such due to clever mechanics and not just because they're giant HP sponges that you'll never outlast in a DPS race, I'm all for it.

I love difficult solo content. Mind, it makes sense why devs would want to focus time and money on group stuff in an MMO, but there is absolutely a place for one-player challenges. Some of the class/job quest duties, for example, can be rather tricky, and that's a good bar to start from.

If these challenges were similarly based in a personal choice made by your character, like job, Grand Company, or role, that could be a good place to start building ideas at.