Lightness and Efficiency are paramount – These are the keys to high performance in all aspects of aeronautics. All high performance aircraft is dependent on this simple concept and should be the defining principle in your design.
K.I.S.S. (Keep It Simple Stupid) – Simple engineering is generally better. In the case of aeronautics; simplicity could also equate to lighter overall weight which equates to a faster plane or more endurance. And with minimal systems in place there is less to go wrong which means you will have more fun time in the air and spend less time making repairs, theoretically.
“Modularity” – Is a concept that we will strive for to future proof our systems and to reduce one-off engineering and for ease for finding replacement parts.
“Two is one, and one is none” (optional) – A wise saying that states that your UAV is only as strong as its weakest link. The weakest link might be unknown until it goes out, so it’s good to have backups. This might sound contrary to KISS because it adds complexity, but think of this as KISS with a backup, and in some scenarios this is the only way to go and in other cases absolutely not the way to go. If you want a fast FPV park flyer then disregard this saying because there is no need for backups and will only slow you down.
A difference between hobbyist and professional large unmanned aircraft is the addition of backup systems. If you want to build a real deal UAV it should have redundancies; everything should have a backup. One simple way to avoid single point failures is to place double of all the normal RC components and have them in parallel independent of each other; essentially two planes worth of electronics and motors, switched by the pilot on the ground and/or automatically transitioned onboard. This would add twice the weight and should only be done on a large wing that can support that type of payload, additionally it’s twice the cost and should only be employed when the UAV is required to perform a critical mission.
TL;DR: At the very least, make it lightweight, efficient and simple. Everything added after that is just for the sake of specific requirements and will have to be considered carefully.