As for now main difficulties are Tourist, Normal and Serious as they add the most reasonable Difficulty curve to the game, changing some strats and making some impossible, also adding some challenge to game other than tricks.

There was a discussion long time ago between me and Freezard, after which we decided that for ILs main difficulties will be Tourist and Serious as the biggest Curve between tricks/survival, though that was about HD series which don't allow to RJ on Tourist, yet it's the easiest difficulty to do harpy/gnaar fly tricks.

For ILs having something inbetween would make no sense, since you can grind it on easiest for speeds and on hardest for challenge (Mental is hardest since difficulty doesn't differ from Hard (not Serious) + enemies become invisible from time to time).

For HD games adding fullgame Normal and Serious makes no sense so far, because they are just much longer. Maybe they will be added under Misc if interest to those categories will appear at all.