More difficulty options would not do anything. People would only play the harder difficulty after a few days anyways.
The only thing I absolutely think would be good is a hard mode for the dungeons, with a difficulty level a bit lower than EX fights and higher than story mode dungeons.
I don't see the point of having something between savage and ultimate. If you find Savage not challenging enough, there is a high chance nothing other than Ultimate will be fulfilling.
That's why Ultimates are such stupidly long multi-phases fights, because the endurance check is basically the challenge on top of the challenge. Without that, it would just be "one more fight" in the raid tier really...
Of course people always want more, so no matter how much they add, people will not have enough.
But I think overall, they should rather invest more time in side content that everybody will participate in, even though some will hate it (like Bozja for example).