Existing Auropilots in SE is still useful. Though I though they would need many improvements in long term. So I thought about this subject for a while and here's my suggestion.
1. Vertical speed hold
On planetary surface flight, ships with hover engines can use this autopilot to help pilots adjust descent/ascent rate.
Pilots can control autopilot by setting vertical speed.
Pilots will set vertical speed by pressing keyboard shortcut or pressing arrow on the display with mouse. Using keyboard shortcut will change vertical speed setting by ±5m/s and doing so while pressing Lalt(or whatever) will change it by ±1m/s.
Another option would be manually input desirable vertical speed with number keys.
Replaces existing hover autopilot(since setting vertical speed to 0m/s will have same effect).
2. Absolute attitude hold
literally. Absolutely keeps ship attitude no matter what situation. Pilots will set desirable attitude by pointing cursor with mouse to target attitude. May replace Killrot autopilot.
3. Relative attitude hold.
When over planetary bodies/orbits, Pilots can keep attitude relative to surface/orbits.
May replace level horizon autopilot.
Pilots will set desirable attitude by controlling Pitch/Roll/Yaw factor by keyboard shortcut.
What makes it different from Absolute attitude hold is that while AAH will never change it's attitude, RAH will change it's attitude. It means that if you are cruising over planetary surface at pitch level 0deg, autopilot will automatically slightly pitch down since planets are round. For orbits, if you set for example +40deg yaw relative to prograde position, ship will keep rotating to keep that relative attitude. This won't happen in AAH.
This means that while AAH will be useful for jobs like docking, interplanetary cruising, RAH will be useful for keeping observing devices towards planet, flying long distance over planetary surface.
Existing Prograde/Retrograde etc orbital position autopilots will be included in RAH as a shortcut icons.
4. Air speed hold.
Useful for long atmospheric flight. Controls main engine thrust to keep airspeed at desirable number. Pilots will control airspeed by using keyboard shortcut; just like any other autopilots.