Any% - Easy, Normal, and Hard
Self Explanatory, brand new account, beat the game as fast as you can. Use RTA timing

Any% - NG+ Easy, Normal and Hard
Beat the game with a previous beaten save. (Meaning you beat the game before, unlocked stuff.) Use RTA timing.
I'd see rules with restarting from checkpoint to reload would be a variable. (Its not that much of a change, the missions where you want more A2G missiles you have a reload line anyway. Hard I could see having it be a hard no.)
Like its faster to restart from checkpoint at the last bit of Mission 20, where you have to get into the Trench Run. The checkpoint is better lined up than the respawn point after the cut scene.

Individual Levels (Might also wanna do something like Easy & Hard? Skipping Medium?)
Use IGT (Which seems to be accurate from start to the mission over, and does not count the post mission RP)

Only missions I'd suggest skipping or ruling differently will be 6 and 11. (Lesser extents being Battle for Farbanti, which is Mission 15, but you then have the Ace fight at the end.)
Both are just flat out 15 minute matches. (You always do 15 minutes, even if you beat it in 5 minutes)
You could in theory include both, if you toss in start the mission, then restart from checkpoint with RTA timer, and finishing the 'run' when you hit the point requirement.

Arsenal Bird%
This one I can see being a bit of weird one. Using Mission 19, you start the mission, then when the Arsenal Bird shows up, you restart from checkpoint (Using the above method for timing 6 and 11) to the final hit of the bird's center part.
I could see this being ran because its a boss fight. Doing it the fastest.

ACE% - Easy and Hard (Skipping Normal)
Literally shoot down all 24 Aces. The issue being that a few missions have multiple Ace spawns. (Mission 12 being a example of 3 Aces. Although, you are capable of shooting down all 3 in that mission without much trouble on Normal.)
This one would be a harder one to time... Maybe RTA and Campaign mode? (Would require NG+). I dunno, this could be discussed more though.

I'm suggesting categories for each difficulty, because not everyone will want to play Hard, or Easy for example. (I couldn't see myself running the full game again on Hard. Even more so hearing its 99.99% missile fired at me.)

Gun Maniac (only machine gun) can be category since it is in the game as a medal and shows you in result screen as a bonus. Plus there is a Mig-21 is the only aircraft with twin machine guns as a special weapon. Only issue is on what difficulty (Hard & Ace have limited ammo)?

Arsenal Bird% doesn't seem like category because it doesn't have it's own mission, its right after other objectives.

As for Arsenal Bird, it is, but its also a boss rush, and its more fun than facing Mr Long name, and hoping for dumb luck hits. That was me just slapping together what could be interesting.

Mihaly Dumitru Margareta Corneliu Leopold Blanca Karol Aeon Ignatius Raphael Maria Niketas A. Shilage
Kappa

I'm sure people will figure out when to hit him, but for first time players, that fight just comes down to "Spam missiles, hope they hit".

best weapons to use are PLSL or EML if you are good with aiming and predicting Mihaly's movement. Plus you can spam QAAM missiles works well too. Also head to head can work too but very risky of getting killed.

I used QAAM and jsut spammed.

But thats neither here nor there, this was a topic for suggestions 😛

What about a F-104 Starfighter category? It would be very different from Any% because of the poor performance and ammo capacity of the aircraft, and would give those who aren't skilled at post-stall manouvers a category they could be competitive in.