... I will give a short answer especially in case of using fixed wing configurations...

At first the most regarding thing are the requirements of your application as for example the resolution you need.

Than this is to combine with the parameters of the sensor you use:

Beginning with resoulution regarding your sensor you have to think about the flight height. You have to check this in combination with minimum speed of your airspeed with the given shutter speed or integration time to have an idea about the resulting image blur (Also depending on angle of view). It could happen that with your combination image blur will be to strong. Than it will be necessary to think about different combinations of focal lenght (changing  optics/lenses) with flight height to get satisfactorilly results ...

Than, thinking about flight strategy knowing your flight height and lens combination you have to consider your angle of view to calulate the needed overlaps between images in and beetwen image stripes...This must fit to your shutter speed/integration time and also with the max. sequence the sensor can be triggered.

At the end of a stripe you have to think about the turn performance of your (fixed wing) airframe. It can be helpful to fly first every second line and coming back, the others...

Under windy conditions it is very helpful only to take images against wind direction.

So the flight line direction can be in the direction of the longest distance of your aerea and under extreme situations also in wind direction.

Considering these you should be able to define your flight plan...

with best regards

Patrick

A design of a path planning from scratch (not just giving GPS waypoints) depends essentially on the dynamics of the system, in the case of UAV (quadcopters or other) the dynamic model has to be well known, with the most parameters that you can get (weight, dimension, interias, different forces, etc...).

Then for sensors, it depends on the navigation area, if it's open, you mostly need a GPS and IMU combined for refining the measurements. If you're in an indoor area, IMU is needed as well, but GPS is inefficient, and in this case you'll need a visual odometry (this can be used outside as well) to estimate the travelling distance of your UAV, and an obstacle avoidance system (sonars, lidars and so on).

The flight height, the focal length of your camera and the pixel size define the GSD (Ground Sample Distance), i.e. the dimension of your pixel on the ground.